Method of on-press developing overcoated lithographic printing plate

ABSTRACT

A method of processing an on-press developable lithographic printing plate involving the removal of overcoat after imagewise exposure and before on-press development is described. The plate comprises a substrate, an on-press ink and/or fountain solution developable photosensitive layer, and an overcoat. The laser imaged plate is stripped off the overcoat, and then under a white room light mounted on press and developed with ink and/or fountain solution. Here the plate is capable of hardening upon exposure to a laser, has limited white light stability before the removal of the overcoat, and has significantly improved white light stability after the removal of the overcoat.

FIELD OF THE INVENTION

This invention relates to lithographic printing plate. Moreparticularly, it relates to a method of on-press developing alithographic plate involving the removal of overcoat before mounting onpress.

BACKGROUND OF THE INVENTION

Lithographic printing plates (after process) generally consist ofink-receptive areas (image areas) and ink-repelling areas (non-imageareas). During printing operation, an ink is preferentially received inthe image areas, not in the non-image areas, and then transferred to thesurface of a material upon which the image is to be produced. Commonlythe ink is transferred to an intermediate material called printingblanket, which in turn transfers the ink to the surface of the materialupon which the image is to be produced.

At the present time, lithographic printing plates (processed) aregenerally prepared from lithographic printing plate precursors (alsocommonly called lithographic printing plates) comprising a substrate anda photosensitive coating deposited on the substrate, the substrate andthe photosensitive coating having opposite surface properties. Thephotosensitive coating is usually a photosensitive material, whichsolubilizes or hardens upon exposure to an actinic radiation, optionallywith further post-exposure overall treatment. In positive-workingsystems, the exposed areas become more soluble and can be developed toreveal the underneath substrate. In negative-working systems, theexposed areas become hardened and the non-exposed areas can be developedto reveal the underneath substrate. Conventionally, the plate is exposedwith an ultraviolet light from a lamp through a separate photomask filmhaving predetermined imaging pattern that is placed between the lightsource and the plate, and the exposed plate is developed with a liquiddeveloper to bare the substrate in the non-hardened or solubilizedareas.

Laser sources have been increasingly used to imagewise expose a printingplate that is sensitized to a corresponding laser wavelength, allowingthe elimination of the photomask film. Suitable lasers include, forexample, infrared lasers (such as laser diode of about 830 nm and NdYAGlaser of about 1064 nm), visible lasers (such as frequency-doubled NdYAGlaser of about 532 nm and violet laser diode of about 405 nm), andultraviolet laser (such as ultraviolet laser diode of about 370 nm).

Laser sensitive plates generally have higher sensitivity (thanconventional plates) because of the limited laser power and the desirefor fast imaging speed. Accordingly, photosensitive plates designed forlaser imaging generally have limited room light stability. For example,before being developed to remove the non-hardened areas,frequency-doubled NdYAG laser sensitive plates usually require red roomlight for handling, violet laser sensitive plates usually require orangeor yellow room light for handling, and infrared laser sensitivephotopolymer plates usually require yellow room light for handling andhave only limited white light stability (due to, for example, the use ofcertain initiator which has spectral sensitivity in the ultravioletregion).

On-press developable lithographic printing plates have been disclosed inthe literature. Such plates can be directly mounted on press afterimagewise exposure to develop with ink and/or fountain solution duringthe initial prints and then to print out regular printed sheets. Noseparate development process before mounting on press is needed,allowing the reduction of labor and the elimination of hazardous waste.Among the patents describing on-press developable lithographic printingplates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449,5,677,110, 5,811,220, 6,014,929, 6,071,675, and 6,482,571.

An on-press developable plate is designed to be developed on alithographic printing press in a pressroom, which is generally underregular office light (white room light). The imagewise exposed platetypically sees the white room light during the handling and on-pressdevelopment. Therefore, the plate must be designed so that it is stableenough under regular office light within a certain time period (such as30 minutes), and yet has sufficient photospeed suitable for imaging withavailable commercial imagers. However, a plate with faster photospeedgenerally has shorter handling window under white room light, while aplate with longer handling window under white room light generally hasslower photospeed. Therefore, it is very difficult to design a lasersensitive on-press developable plate having both fast enough photospeedand wide enough handling window (in terms of time) under white roomlight.

It would be desirable if a laser sensitive on-press developablelithographic plate and/or method of using it can be designed so that theplate has sufficient photospeed during laser exposure and the laserexposed plate can be handled freely under white room light before andduring on-press development.

The inventor has found that such a desire can be achieved by a method ofremoving the overcoat of a specific overcoated lithographic plate afterimagewise exposing said plate and before mounting said plate on pressfor development with ink and/or fountain solution. Here said plate iscapable of hardening upon exposure to a laser and has limited white roomlight stability before the removal of the overcoat, and hassignificantly improved white room light stability after the removal ofthe overcoat.

SUMMARY OF THE INVENTION

According to the present invention, there has been provided a method oflithographically printing images on a receiving medium, comprising inorder:

-   -   (a) providing a lithographic plate comprising (i) a        substrate, (ii) a photosensitive layer, and (iii) an overcoat;        said photosensitive layer being soluble or dispersible in ink        and/or fountain solution and capable of hardening upon exposure        to a laser having a wavelength selected from 200 to 1200 nm;    -   (b) imagewise exposing said plate with said laser to cause        hardening of said photosensitive layer in the exposed areas;    -   (c) removing said overcoat from said plate;    -   (d) mounting said plate onto a lithographic press; and    -   (e) contacting said plate with ink and/or fountain solution on        said lithographic press to remove the photosensitive layer in        the non-hardened areas and to lithographically print images from        said plate to the receiving medium;    -   (f) wherein said mounting and on-press development (steps d        and e) are performed with the plate under a white room light,        said photosensitive layer before the removal of the overcoat is        capable of hardening or causing background toning under said        room light for less than a time period selected from 1 to 60        minutes, and said photosensitive layer after the removal of the        overcoat is incapable of hardening or causing background toning        under said room light for at least twice of said time period.

The plate is imagewise exposed with a laser on a laser imager, strippedoff the overcoat, and then mounted on press for development with inkand/or fountain solution and lithographic printing, with the plate beingunder a white room light during the mounting and on-press development aswell as lithographic printing. The plate has limited white room lightstability before removing the overcoat and has significantly improvedwhite room light stability after removing the overcoat. While thepresence of the overcoat during laser imaging allows sufficiently fastphotospeed, the overcoat removal of the laser exposed plate allows widerwindow (in terms of time) for handling the plate under while room lightduring and right before on-press development. The overcoat-removed plateis preferably inspected by the operator under white room light beforemounting on press for development with ink and/or fountain solution.

Preferably, said photosensitive layer before the removal of the overcoatis capable of hardening or causing background toning under said whiteroom light for less than 60 minutes (more preferably less than 30minutes, even more preferably less than 10 minutes, and most preferablyless than 1 minute), and said photosensitive layer after the removal ofthe overcoat is incapable of hardening or causing background toningunder said white room light for at least 120 minutes (more preferably atleast 300 minutes and most preferably any amount of time). Here thebackground toning (which is the presence of unclean or slightly inkednon-imaging areas for a developed plate on press) is caused by partialhardening of the photosensitive layer after exposure to said white roomlight for certain amount of time.

The overcoat can be removed by any means, preferably by mechanicalpeeling or by rinsing with water or an aqueous solution. For overcoatinsoluble or non-dispersible in water or an aqueous solution, theovercoat is preferably removed by peeling. For plate rinsed with wateror an aqueous solution, the overcoat is soluble or dispersible in wateror an aqueous solution, preferably soluble in water. The plate rinsedwith water or an aqueous solution to remove the overcoat is preferablyfurther dried with forced air and/or heat (preferably forced hot air) todry off any wet-looking liquid layer on the plate surface beforemounting on press.

The photosensitive layer is negative-working, capable of hardening uponexposure to a laser having a wavelength selected from 200 to 1200 nm,preferably an infrared laser of from 750 to 1200 nm or a violet orultraviolet laser of from 200 to 430 nm. A preferred photosensitivelayer comprises a polymeric binder, a free radical polymerizablemonomer, a free radical initiator, and a sensitizing dye.

The laser exposure and overcoat removal (steps b to c) can be performedwith the plate under a yellow or red light (such as in a room withyellow or red light), in darkness or substantial darkness (such as withthe plate in an imager having light-tight covers and then in an overcoatremoval device having light-tight covers), or under white room light forlimited time, depending on the white light sensitivity of the plate.Preferably, the laser exposure and overcoat removal are performed withthe plate under a yellow or red light, or in darkness or substantialdarkness. The overcoat removal (step c) is preferably performed on anovercoat removal device that is connected to or is part of an imagingdevice for the laser exposure (step b).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lithographic plate of this invention comprises on a substrate aphotosensitive layer and an overcoat. The overcoat is capable of beingremoved prior to mounting the plate on press for development with inkand/or fountain solution and lithographic printing. The overcoat ispreferably removed by mechanical peeling or by rinsing with water or anaqueous solution. The overcoat preferably has a coverage of at least 0.1g/m², more preferably from 0.5 to 10 g/m², even more preferably from 1.0to 7 g/m², and more preferably from 2.0 to 5.0 g/m².

The overcoat can be any film-forming material which is substantiallytransparent to the imaging laser and is capable of being removed bymechanical peeling or by rinse with water or an aqueous solution. Theovercoat can be formed on the plate by coating from a solution ordispersion or by laminating from a polymeric film, preferably by coatingfrom a solution or dispersion containing a film-forming polymer, morepreferably by coating from an aqueous solution or dispersion containinga film-forming polymer. Here the overcoat removable by rinsing withwater or an aqueous solution must be soluble or dispersible in water oran aqueous solution, either said overcoat being formed by coating from asolution or dispersion or by lamination from a polymeric film. Thecoating can be performed by any known method, such as roller coating,slot coating, curtain coating, Mayer rod coating, dip coating, or spraycoating; preferably roller coating or slot coating. The coated overcoatis further dried to remove the water as well as any solvent, preferablyby forced hot air drying, radiation drying, or combination of forced airdrying and radiation drying.

The solution or dispersion containing a film-forming polymer suitablefor forming overcoat can be a water soluble polymer solution (containingwater soluble polymer) or polymer dispersion (including polymer emulsionand latex, containing water-insoluble polymer, with or without additionof water soluble polymer), preferably a water soluble polymer solution.A suitable water soluble polymer overcoat comprises a water-solublepolymer, such as polyvinyl alcohol (including various water-solublederivatives of polyvinyl alcohol). Combination of two or morewater-soluble polymers (such as a combination of polyvinyl alcohol andpolyvinylpyrrolidone) can also be used. Polyvinyl alcohol is a preferredwater-soluble polymer. Various additives, such as surfactant, wettingagent, defoamer, leveling agent and dispersing agent, can be added intothe overcoat formulation to facilitate, for example, the coating ordevelopment process. Examples of surfactants useful in the overcoat ofthis invention include polyethylene glycol, polypropylene glycol, andcopolymer of ethylene glycol and propylene glycol, polysiloxanesurfactants, perfluorocarbon surfactants, alkylphenyl ethylene oxidecondensate, sodium dioctylsulfosuccinate, sodiumdodecylbenzenesulfonate, and ammonium laurylsulfate. Various organic orinorganic micro particles may be added into the overcoat to, forexample, reduce the tackiness or moisture sensitivity of the plate.

The polymeric film suitable for laminating onto the photosensitive layerto form the overcoat can be any polymeric film, such as polymer filmsbased on polyester (including polyethylene terephthalate), polyethylene,polypropylene, ethylene/propylene copolymer, polystyrene, polyvinylbutyrol, polynitrile, polyamide, polyimide, polyvinyl chloride,polycarbonate, polymethylmethacrylate, acrylate/styrene copolymer,methacrylate/styrene copolymer, polyurethane, epoxy-amine polymer,nylon, Teflon, cellulose and its film-forming derivatives, polyvinylacetate, and polyvinyl alcohol. Polyester, polyethylene, andpolypropylene are preferred polymeric film; and polyester is morepreferred. An overcoat formed from a film that is insoluble andnon-dispersible in water or an aqueous solution is suitable for removalby mechanical peeling, but not suitable for removal by rinsing withwater or an aqueous solution.

The polymeric film can be directly laminated onto the photosensitivelayer of a plate. Alternatively, the polymeric film can be pre-coatedwith a layer of adhesive before laminating onto the photosensitive layerof a plate (with the adhesive layer facing the photosensitive layer).Here the adhesive layer can be made of any adhesive material; preferablya pressure sensitive adhesive, a heat sensitive adhesive, aradiation-sensitive adhesive, or a moisture sensitive adhesive; morepreferably a pressure sensitive adhesive. Preferably, the polymeric filmis pre-coated with a layer of adhesive (more preferably a pressuresensitive adhesive) before laminating onto the photosensitive layer ofthe plate. The adhesive layer preferably has stronger adhesion to thefilm than to the photosensitive layer so that such adhesive layer comesoff with the film during mechanical peeling.

The polymeric film can be laminated onto the photosensitive layer of aplate by any means, preferably by a pair of laminating rollers, with theplate at room temperature or at elevated temperature, preferably atelevated temperature. When laminating the polymeric film to thephotosensitive layer of the plate at an elevated temperature, a pair ofheated laminating rollers is preferably used.

For plate suitable for removing the overcoat by mechanical peeling, theovercoat can be peeled off as one or more large sheets, or as many smallpieces. The overcoat can be peeled off from the plate by any means. Thepeeled away overcoat can be substantially free of any photosensitivelayer, or can be attached with some of the photosensitive layer (i.e.,part of the photosensitive layer, especially in the non-hardened areas,may be carried away with the overcoat). The mechanical peeling of theovercoat can be performed at room temperature or an elevatedtemperature, preferably at room temperature. The overcoat can either besoluble or dispersible in water or be insoluble and non-dispersible inwater. In the manufacture of the original plate, the overcoat can becoated onto the photosensitive layer from a solution or dispersion,preferably an aqueous solution, or can be laminated onto thephotosensitive layer from a film, preferably a polymeric film.

The overcoat suitable for being peeled off as one or more large sheetsis preferably strong enough so that the overcoat can be peeled off asone or more thin films, more preferably one thin film. Preferably, it ispeeled off by pulling with mechanical device or by hand from one end,preferably by a mechanical device. The mechanical device can be clampedto the overcoat at one end, adhered to the overcoat surface, or suckedby vacuum on the overcoat surface.

The overcoat suitable for being peeled off as many small pieces ispreferably capable of being broken apart by mechanical scratching,rubbing or brushing, or by air blowing or vacuum suction. Preferably,the overcoat on the plate is brushed with a brush which is capable ofbreaking apart and peeling off the overcoat without damaging thephotosensitive layer in the hardened areas. The peeled off overcoatpieces can be removed from the plate by vacuum suction, air blowing,brushing, rubber blade wiping, or other mechanical action, preferably byvacuum suction. The peeled off overcoat pieces can be collected in awaste vacuum bag or a container.

For plate suitable for removing the overcoat by rinsing with water or anaqueous solution, the overcoat is soluble or dispersible in water or anaqueous solution. The overcoat can be removed by contacting with wateror an aqueous solution, with or without brushing or other rubbing action(by a brush, cloth, roller, or the like) or agitation (by stirring,vibrating, ultrasonifying, spraying with, or flushing with the water oraqueous solution). The water or aqueous solution can be applied to theplate by any means, such as by passing the plate through a bathcontaining rinsing water or aqueous solution or by spraying or pouringthe water or aqueous solution to the plate which passes through. Theplate rinsed with water or an aqueous solution can be further dried todry off the plate surface, preferably by forced hot air.

While a plate is generally designed for overcoat removal with one of themethods, such as mechanical peeling or rinsing with water or an aqueoussolution, the plate can also be designed in a way so that it is capableof removing the overcoat by both mechanical peeling and rinsing withwater or an aqueous solution. Such a plate should have an overcoat whichis soluble or dispersible in water or an aqueous solution and is alsocapable of being peeled off mechanically.

For plate suitable for removing the overcoat by rinsing with water or anaqueous solution, either water or an aqueous solution can be used,depending on the particular overcoat as well as the plate. Preferably,an aqueous solution which is capable of improving certain performancecharacteristics of the plate is used. More preferably, the aqueoussolution is capable of improving the hydrophilicity of the substrate.The water or aqueous solution can be at room temperature or an elevatedtemperature, preferably at room temperature.

The water can be any water, such as tap water and deionized water,preferably tap water. The water after rinse may be reused for rinsingadditional plates. The reused water may be filtered to remove any soliddebris.

The aqueous solution can be any water-based solution capable of removingthe overcoat without causing any adverse effect to the plate.Preferably, the aqueous solution is capable of improving certainperformance of the plate, in addition to removing the overcoat toimprove the white room light stability, such as improving thehydrophilicity of the substrate, forming or enhancing the visible imagesof the laser exposed plate, improving the developability of the plate,and/or further improving the white light stability of the photosensitivelayer in the non-hardened areas. In other words, such an aqueoussolution is preferably also a deactivating solution capable ofdeactivating the photosensitive layer, a discoloring solution capable ofchanging the color of the photosensitive layer primarily or only in thenon-hardened areas, a hydrophilizing solution capable of enhancing thehydrophilicity of the substrate, and/or a development enhancing solutioncapable of increasing the on-press developability with ink and/orfountain solution. More preferably, the aqueous solution is awater-based solution comprising a deactivating agent, a discoloringagent, a hydrophilizing agent, or a development enhancer.

The aqueous solution preferably comprises at least 50% by weight ofwater, most preferably at least 80% by weight of water. One or morewater-soluble organic solvents, such as ethylene glycol, can be addedinto the aqueous solution. Certain additives, such as dye, dispersedpigment, bactericide, stabilizer, reducer, thickening agent, andsurfactant, can be added.

The deactivating agent can be any material that can deactivate the photohardening capability of the photosensitive layer in the non-hardenedareas, so that the non-hardened photosensitive layer (which isoriginally capable of hardening under a room light) becomes incapable orhaving reduced rate (preferably incapable) of hardening under such roomlight. The deactivating agent can be a solid, liquid, or gas; preferablya liquid or solid. Either organic or inorganic compound can be used asdeactivating agent, such as organic or inorganic acid, base, oxidizer,reducer, or inhibitor. Various deactivating agents have been describedin U.S. Pat. No. 7,213,516, and U.S. patent application Ser. Nos.11/356,911, 11/728,648, 11/787,878, and 11/800,634; the entiredisclosures of which are hereby incorporated by reference. Thedeactivating agent is preferably soluble in water and is applied from anaqueous solution. Various additives, such as surfactant, stabilizer,bactericide, defoamer, dye, cosolvent, pigment, and thickener can beadded in the discoloring solution. The concentration of the deactivatingagent in a deactivating solution is from 0.01 to 70%, more preferablyfrom 0.1 to 30%, and most preferably from 1 to 10% by weight of thesolution.

For free radical polymerizable photosensitive layer, the deactivatingagent can be a compound that can react with a component of the freeradical initiating system (such as initiator, sensitizing dye, hydrogendonor, or monomer; preferably the initiator, sensitizing dye, orhydrogen donor). For cationic polymerizable photosensitive layer, thedeactivating agent can be a compound that can react with a component ofthe cationic polymerization system (such as the initiator which is anacid generator, sensitizing dye, or monomer; preferably the initiator orsensitizing dye).

For polymerizable photosensitive layer having an amine group or otheracid-reacting group (a group capable of reacting with an acid) in theinitiator, sensitizing dye, or hydrogen donor, an acid compound(including organic acid and inorganic acid) can be used as thedeactivating agent. Suitable organic acids include, for example, organiccompounds having at lease one carboxylic acid group, sulfonic acidgroup, or phosphonic acid group. Suitable inorganic acids include, forexample, phosphoric acid, boric acid, and hydrochloride acid. Preferredacids are those with moderate acidity, such as organic compounds with atleast one carboxylic acid group, phosphoric acid, polyvinyl phosphonicacid, and boric acid. More preferred are water soluble organic acids.Most preferred are water-soluble organic compounds having at least onecarboxylic acid group. Suitable organic acids include, for example,citric acid, acetic acid, salicylic acid, glycolic acid, malic acid, andlactic acid. Citric acid and malic acid are particularly suitablebecause they are widely used natural organic acids and are non-hazardousto the environment. The acid is preferably applied as an aqueoussolution to deactivate the photosensitive layer. When strong acid (suchas hydrochloric acid) is used as deactivating agent, it is preferablydiluted to low concentration (such as less than 0.5%, preferably lessthan 0.1% by weight) in an aqueous solution to apply to the plate sothat it does not damage the plate or cause safety problem. The acidicdeactivating solution preferably has a pH of from 0.1 to 6.5, morepreferably from 0.5 to 5.0, and most preferably from 1.0 to 4.0. Theacidic deactivating solution preferably has a concentration of from 0.01to 70%, and more preferably from 0.05 to 30% by weight of the solution.The aqueous acidic deactivating solution based on an organic acidpreferably has a concentration of from 0.1 to 70%, more preferably from0.5 to 30%, and most preferably from 2 to 10% by weight of the solution.

An alkaline compound can also be used as the deactivating agent forcertain negative plates with free radical or cationic polymerizable orother acid crosslinkable photosensitive layers because it can react withcertain free radical initiating system (such as initiator, sensitizingdye, or hydrogen donor), certain cationic initiating system (such asinitiator which is an acid generator, or sensitizing dye), and otheracid crosslinkable systems (such as negative-working diazonaphthoquinonesystems). For example, an alkaline compound can react with an ionicinitiator such as an onium salt, an ionic sensitizing dye such as acyanine dye, or a hydrogen donor having carboxylic acid or thiol group;and can also neutralize with a cationic initiator which is an acidgenerator. Suitable alkaline compounds include, for example, sodiumsilicate, potassium silicate, sodium carbonate, sodium hydroxide, andorganic amines. Preferred alkaline compounds are water-soluble compoundswith moderate basicity, such as sodium silicate, potassium silicate,ammonium hydroxide, and amines. More preferred amines are organicamines, including polymeric amines. Suitable water-soluble aminesinclude regular amine compounds such as triethylamine, triethanolamine,2-amino-2-methyl-1-propanol, tris(hydroxymethyl)aminomethane andN-methyl-2-pyrrolidone, and polymeric amines such as polyethyleneamine.The alkaline compound is preferably applied as an aqueous solution todeactivate the photosensitive layer. When strong base (such as sodiumhydroxide) is used as deactivating agent, it is preferably diluted tolow concentration (such as less than 0.5%, preferably less than 0.1% byweight) in an aqueous solution so that it does not damage the plate orcause safety problem. The alkaline deactivating solution preferably hasa pH of from 7.5 to 13.5, more preferably from 8.0 to 12.0, and mostpreferably from 9.0 to 11.0. The alkaline deactivating solutionpreferably has a concentration of from 0.01 to 70%, and more preferablyfrom 0.05 to 30% by weight of the solution. The aqueous alkalinedeactivating solution based on organic amine preferably has aconcentration of from 0.1 to 70%, more preferably from 0.5 to 30%, andmost preferably from 2 to 10% by weight of the solution.

A free radical inhibitor can be used as the deactivating agent forplates with a free radical polymerizable photosensitive layer. Examplesof suitable free radical inhibitors include methoxyhydroquinone,hydroquinone, 2,6-di-tert-butyl-4-methylphenol, polyvinylphenol, othercompounds with at least one phenol group, and various commercial freeradical stabilizer. Preferably, the inhibitor is dissolved in water or awater-solvent mixture (containing water and a water soluble organicsolvent) to form an aqueous deactivating solution for applying to theplate. The deactivating solution based on a free radical inhibitorpreferably has a concentration of from 0.1 to 70%, more preferably from0.5 to 30%, and most preferably from 2 to 10% by weight of the solution.

The discoloring agent suitable for this invention can be any materialthat is capable of changing the color of the photosensitive layerprimarily or only in the non-hardened areas, with less or no colorchange in the hardened areas. Preferably, the discoloring agent suitablefor this invention is a material that is capable of reacting with a dyeor latent dye in the photosensitive layer to cause color change(including turning on or turning off of color). The discoloring agent ispreferably soluble in water and is dissolved in water to form an aqueousdiscoloring solution. The discoloring agent can be a solid, liquid, orgas: preferably a liquid or solid. Various additives, such assurfactant, stabilizer, bactericide, defoamer, dye, cosolvent, pigment,and thickener can be added in the discoloring solution. Depending on thephotosensitive layer as well as its dye or pigment system, thediscoloring agent can be different.

The discoloring solution is capable of diffusing into the non-hardenedareas more efficiently than into the hardened areas, the application ofsuch discoloring solution causes color change primarily or only in thenon-hardened areas, with less or no color change in the hardened areas.Here, the term “the discoloring solution is capable of diffusing intothe non-hardened areas of the photosensitive layer more efficiently thaninto the hardened areas” means that more discoloring solution candiffuse into the non-hardened areas of the photosensitive layer whileless or no discoloring solution can diffuse into the hardened areas ofthe photosensitive layer.

For photosensitive layer comprising a visible dye capable ofdiscoloration, the discoloring agent can be any compound capable ofdiscoloring the dye. The application of the discoloring agent from adiscoloring solution changes the color of the dye primarily or only inthe non-hardened areas, with less or no color change in the hardenedareas. Preferably, the color change is color reduction. For example, animagewise exposed photosensitive layer comprising a crystal violet canbe discolored with a hydrochloric acid aqueous solution to reduce theblue color in the non-exposed areas, with the hardened areas remainingsubstantially the original blue color.

For photosensitive layer comprising a latent dye, the discoloring agentcan be any compound capable of turning on the color of the latent dye.The application of said discoloring agent from a discoloring solutionpartially or completely turns on the color of the latent dye primarilyor only in the non-hardened areas, with less or no color change in thehardened areas. Examples of such system include a photosensitive layerhaving an acid sensitive latent dye and a discoloring agent which is anacid in an aqueous solution, and the application of such acid solutionto the imagewise exposed plate turns on the color primarily or only inthe non-hardened areas, with less or no color turning on in the hardenedareas. Various latent dyes can be used, such as leuco crystal violet,leucomalachite green, azobenzene, 4-phenylazodiphenylamine, andmethylene blue dyes.

For photosensitive layer having a visible dye capable of changing color(preferably turning off color) or a latent dye capable of turning oncolor upon contact with an acid, an acid aqueous solution can be used asthe discoloring solution. The acid can be an organic acid or inorganicacid. Suitable organic acids include, for example, organic compoundshaving at lease one carboxylic acid group, a sulfonic acid group, orphosphonic acid group. Suitable inorganic acids include, for example,phosphoric acid, boric acid, and hydrochloride acid. Preferred acids arethose with moderate acidity, such as organic compounds with at least onecarboxylic acid group, phosphoric acid, polyvinyl phosphonic acid, andboric acid. More preferred are water soluble organic acids. Mostpreferred are water-soluble organic compounds having at least onecarboxylic acid group. Solid acid (such as citric acid) is particularuseful because it does not evaporate from the photosensitive layer.Suitable organic acids include, for example, citric acid, acetic acid,salicylic acid, glycolic acid, malic acid, and lactic acid. Citric acidand malic acid are particularly suitable because they are widely usednatural organic acids and are non-hazardous to the environment. The acidis preferably applied as an aqueous solution to discolor thephotosensitive layer. When strong acid (such as hydrochloric acid) isused as discoloring agent, it is preferably diluted to low concentration(such as less than 0.5%, preferably less than 0.1% by weight) in anaqueous solution to apply to the plate so that it does not damage theplate or cause safety problem. The acidic discoloring solutionpreferably has a pH of from 0.1 to 6, more preferably from 0.5 to 4.0,and most preferably from 1.0 to 3.0. The acidic discoloring solutionpreferably has a concentration of from 0.01 to 70%, and more preferablyfrom 0.05 to 30% by weight of the solution. The aqueous acidicdiscoloring solution based on an organic acid preferably has aconcentration of from 0.1 to 70%, more preferably from 0.5 to 30%, andmost preferably from 2 to 10% by weight of the solution.

For photosensitive layer having a visible dye capable of changing color(preferably turning off color) or a latent dye capable of turning oncolor upon contact with a base, an alkaline aqueous solution can be usedas the discoloring solution. Suitable alkaline compounds include, forexample, sodium silicate, potassium silicate, sodium carbonate, sodiumhydroxide, and organic amines. Preferred alkaline compounds arewater-soluble compounds with moderate basicity, such as sodium silicate,potassium silicate, ammonium hydroxide, and amines. More preferredamines are organic amines, including polymeric amines. Suitablewater-soluble amines include regular amine compounds such astriethylamine, triethanolamine, 2-amino-2-methyl-1-propanol,tris(hydroxymethyl)aminomethane and N-methyl-2-pyrrolidone, andpolymeric amines such as polyethyleneamine. The alkaline compound ispreferably applied as an aqueous solution to discolor the photosensitivelayer. When strong base (such as sodium hydroxide) is used asdiscoloring agent, it is preferably diluted to low concentration (suchas less than 0.5%, preferably less than 0.1% by weight) in an aqueoussolution so that it does not damage the plate or cause safety problem.The alkaline discoloring solution preferably has a pH of from 8 to 13.5,more preferably from 8.5 to 12.0, and most preferably from 9.0 to 11.0.The alkaline discoloring solution preferably has a concentration of from0.01 to 70%, and more preferably from 0.05 to 30% by weight of thesolution. The aqueous alkaline discoloring solution based on an organicamine preferably has a concentration of from 0.1 to 70%, more preferablyfrom 0.5 to 30%, and most preferably from 2 to 10% by weight of thesolution.

For a photosensitive layer comprising a dispersed pigment, thediscoloring solution can be any aqueous solution capable of causingflocculation of the dispersed pigment primarily or only in thenon-hardened areas. Such a discoloring solution is also calledflocculating solution. Here the term flocculation means becomingnon-dispersed, aggregated, or insolubilized from a dispersed orsolubilized system. Preferably, the flocculating solution is an aqueoussolution (including blend of water and an organic solvent) capable ofcausing flocculation (or aggregation) of the dispersed pigment in thephotosensitive layer upon diffusing into it and capable of diffusinginto the non-hardened areas of the photosensitive layer more efficientlythan into the hardened areas. More preferably, the flocculating solutiondiffuses into the photosensitive layer in the non-hardened areas toflocculate the pigment without completely dissolving the photosensitivelayer so that the photosensitive layer in the non-hardened areas doesnot flow around. Any pigment can be used, including organic pigment suchas copper phthalocyanine and other phthalocyanine pigments, andinorganic pigment such as iron oxide and copper carbonate. The pigmentis dispersed as fine particles in the photosensitive layer, usually withcertain pigment dispersant or polymer, so as to have good colorstrength. The flocculating solution is preferably capable of swelling(without completely dissolving) the photosensitive layer in thenon-hardened areas but incompatible with (causing flocculation of) thepigment dispersion. A compound capable of causing or helping theflocculation (such as by physical interaction or chemical reaction withthe dispersant) can be used in the flocculating solution.

For a photosensitive layer comprising a visible dye capable offlocculation, the discoloring solution (also called flocculatingsolution here) can be any aqueous solution capable of causingflocculation of such dye primarily or only in the non-hardened areas.The visible dye is insoluble in the non-hardened photosensitive layersoaked with such flocculating solution and is capable of flocculatinginto less or different colored (preferably less colored) aggregates inthe photosensitive layer. Preferably, the flocculating solution iscapable of causing flocculation of the visible dye in the photosensitivelayer upon diffusing into it and capable of diffusing into thenon-hardened areas of the photosensitive layer more efficiently thaninto the hardened areas. More preferably, the flocculating solution iscapable of diffusing into the photosensitive layer in the non-hardenedareas without completely dissolving the photosensitive layer so that thephotosensitive layer in the non-hardened areas does not flow around.

The hydrophilizing agent can be any water-soluble compound capable ofenhancing the hydrophilicity of the substrate. Preferably, suchhydrophilizing agent is an acid or base, more preferably an acid.Suitable acid compounds include organic compounds with at least onecarboxylic acid groups, polymers with phosphonic acid groups, andphosphoric acid. Particularly suitable acid compounds include citricacid, acetic acid, salicylic acid, glycolic acid, malic acid, lacticacid, phosphoric acid, and polyvinyl phosphonic acid. The hydrophilizingagent is preferably dissolved in water and/or an organic solvent, morepreferably in water, to form a hydrophilizing solution for applying tothe plate.

The development enhancer can be any water soluble compound capable ofenhancing the developability of the photosensitive layer in ink and/orfountain solution. Preferably, such development enhancer is an organiccompound capable of enhancing the developability of the photosensitivelayer in ink and/or fountain solution. More preferably, such developmentenhancer is a liquid organic compound capable softening thephotosensitive layer. The water soluble liquid organic compound suitableas development enhancer preferably has a boiling point of at least 150°C., more preferably at least 200° C., and most preferably at least 250°C. Suitable development enhancers include, for example, polyethyleneglycol, glycerin, methoxypropanol, diethyleneglycol, triethyleneglycol,and various nonionic surfactants. The development enhancer is dissolvedin water to form a development enhancing solution for applying to theplate. Preferably, the application of the development enhancer increasesthe ease of developing the plate, so that the roll up impressions (thenumber of rotations of the plate cylinder between engaging the inkingroller and completely cleaning up the background of the plate) isreduced by at least 5 impressions, more preferably at least 10impressions, and most preferably at least 20 impressions due to theapplication of the development enhancer. For example, for an untreatedplate originally requiring 30 roll up impressions (of the rotations ofthe plate cylinder of a lithographic press) to achieve clean background,the same plate treated with a development enhancer only requires at most25 roll up impressions (preferably at most 20 impressions, morepreferably at most 10 impressions) to achieve clean background.

The aqueous solution suitable for this invention can be any aqueoussolution capable of removing the overcoat without causing adverse effectto the plate. Preferably, the aqueous solution is capable of enhancing acertain aspect of the performance of the plate. More preferably, theaqueous solution is capable of enhancing more than one aspects of theperformance of the imagewise exposed plate, such as enhancing both thewhite light stability (by deactivating the photosensitive layer) and thevisible image contrast (by discoloration primarily or only in thenon-hardened areas). Most preferably, the aqueous solution is capable ofenhancing the white light stability (by deactivation), the imagecontrast (by discoloration primarily or only in the non-hardened areas),the hydrophilicity of the substrate, and the developability of thephotosensitive layer. The aqueous solution capable of two or morefunctions (such as both deactivation and discoloration) can comprise 2separate components (such as a deactivating agent and a discoloringagent), can comprise the same component capable of both functions (suchas a deactivating agent which is also a discoloring agent), or cancomprise both a component with two or more functions and a componentwith only single function. Preferably, the aqueous solution comprises atleast one component which is capable of two or more functions (such asboth deactivating the photosensitive layer and hydrophilizating thesubstrate). Examples of multifunctional components include an acidcompound (such as citric acid, applied from an aqueous solution) whichis capable of deactivation, discoloration, and hydrophilization forcertain plates. Examples of single-functional components include a watersoluble organic solvent (such as triethyleneglycol, applied from anaqueous solution) which is capable of enhancing the on-pressdevelopability of the photosensitive layer.

The rinsing of a laser exposed plate with water or an aqueous solutionis preferably performed on a rinsing device (which is a type of overcoatremoval devices of this application). More preferably, the plate istransported through the rinsing device to contact with water or anaqueous solution. The rinsing device preferably further comprises adrying unit (such as a pair of squeegee rollers, forced air, heater, ora pair of squeegee rollers followed by forced hot air) to dry off theliquid on the plate surface. The rinsing device can be a separate devicefrom the exposure device, or can be connected to or be part of theexposure device. Preferably, the rinsing device is connected to or ispart of the exposure device.

The plate rinsed with water or an aqueous solution is preferably furtherdried to remove at least some of the water from the aqueous solution(which has been applied to the plate). More preferably the plate isdried so that the plate surface is at least free of overflowing liquid.Most preferably, the plate is dried so that the plate surface is free ofany wet-looking liquid layer. The rinsed plate can be dried by anymeans, such as squeegee rollers, forced air, heater, or rubber blade.Preferably, the rinsed plate is dried with squeegee rollers, forced hotair, or squeegee rollers followed by forced air. More preferably, therinsed plate is dried by forced hot air. Most preferably, the rinsedplate is dried by a pair of squeegee rollers followed by forced hot air.

The plate stripped off overcoat (by mechanical peeling or by rinsingwith water or an aqueous solution) may be further overall exposed with asecond radiation to, for example, further enhance the visible image orgive a different-colored visible image and/or to cause furthercrosslinking of the photosensitive layer in the hardened areas, withoutcausing hardening of the non-hardened areas, before on-pressdevelopment. The radiation for the overall exposure can be applied withany exposure method which delivers radiation to the whole photosensitivelayer; preferably from a lamp on an exposure device or on a treatingdevice (by passing through it); more preferably by passing through aradiation from a lamp on a treating device. Here the radiation asapplied is incapable of causing hardening of the particularphotosensitive layer without overcoat.

The plate stripped off overcoat may be further overall heated to, forexample, further enhance the visible image or give a different-coloredvisible image and/or to cause further crosslinking of the hardened areasof the photosensitive layer, before on-press development. The heatingcan be applied through any means, such as hot plate, oven,passing-through heater, radiation heater, and hot air heater.Preferably, the plate is passed through a heater having radiation or hotair. More preferably, the plate is passed through a heater which is partof or connected to an overcoat removing device.

The laser imaged plate may be overall heated to an elevated temperaturebefore removing the overcoat. Such a heating is called preheat, and maybe utilized to cause further crosslinking of the hardened areas of thephotosensitive layer. The heating can be applied through any means, suchas hot plate, oven, passing-through heater, radiation heater (with aradiation which does not cause hardening of the photosensitive layer inthe non-hardened areas), and hot air heater. Preferably, the plate ispassed through a heater having radiation or hot air. More preferably,the plate is passed through a heater which is part of or connected tothe laser imager.

For plate which is stripped off the overcoat by mechanical peeling, orby rinse with water or an aqueous solution followed by drying withforced air or heat to remove any wet surface, the laser imaged andovercoat-removed plate can he directly mounted on press for developmentwith ink and/or fountain solution, or can be stacked together with otherdried plates before mounting on-press for development. Preferably, morethan one plates are stacked together, and then picked up by the operatorto mount on press for development with ink and/or fountain solution andlithographic printing.

The plate of the instant invention (with overcoat) can be supplied assheets or roll, preferably as stack of sheets. The plate sheets or rollmay or may not have interleafing paper in between the plates.Preferably, the overcoat is durable and non-tacky enough so that nointerleaving paper is required between plates when supplied. This willsimplify the handling of the plate, especially during mechanical feedingof the plate to the laser imager. Because the overcoat does not need tobe removed on press by the instant invention, it allows the opportunityto easily design an overcoat which is durable and non-tacky enoughsuitable for handling without the use of interleafing paper; preferably,such an overcoat is incapable of removal after contacting with inkand/or fountain solution on press for up to 200 rotations of the platecylinder.

The substrate employed in the lithographic plates of this invention canbe any lithographic support. Such a substrate may be a metal sheet, apolymer film, or a coated paper. Aluminum (including aluminum alloy)sheet is a preferred metal support. Particularly preferred is analuminum support that has been grained and anodized (with or withoutdeposition of a barrier layer). Polyester film is a preferred polymericfilm support. A surface coating may be coated to achieve desired surfaceproperties. For wet plate, the substrate should have a hydrophilic oroleophilic surface, depending on the surface properties of thephotosensitive layer (preferably with opposite philicity to thesubstrate); more preferably, a wet lithographic plate has a hydrophilicsubstrate and an oleophilic photosensitive layer. For waterless plate,the substrate should have an oleophilic or oleophobic surface, dependingon the surface properties of the photosensitive layer (preferably withopposite philicity to the substrate).

Particularly suitable hydrophilic substrate for a wet lithographic plateis an aluminum support that has been grained and anodized; such asubstrate is preferably further deposited with a hydrophilic barrierlayer. Surface graining can be achieved by mechanical graining orbrushing, chemical etching, and/or AC electrochemical graining. Theroughened surface can be further anodized to form a durable aluminumoxide surface using an acid electrolyte such as sulfuric acid and/orphosphoric acid. The roughened and anodized aluminum surface can befurther thermally or electrochemically coated with a layer of silicateor hydrophilic polymer such as polyvinyl phosphonic acid,polyacrylamide, polyacrylic acid, polybasic organic acid, copolymers ofvinyl phosphonic acid and acrylamide to form a durable hydrophiliclayer. Polyvinyl phosphonic acid and its copolymers are preferredpolymers. Processes for coating a hydrophilic barrier layer on aluminumin lithographic plate application are well known in the art, andexamples can be found in U.S. Pat. Nos. 2,714,066, 4,153,461, 4,399,021,and 5,368,974. Suitable polymer film supports for a wet lithographicplate include a polymer film coated with a hydrophilic layer, preferablya hydrophilic layer that is crosslinked, as described in U.S. Pat. No.5,922,502.

For the plate of this application, at least the hardened areas of thephotosensitive layer exhibit an affinity or aversion substantiallyopposite to the affinity or aversion of the substrate to at least oneprinting liquid selected from the group consisting of ink and anadhesive fluid for ink (including both plates with non-phase-switchablephotosensitive layer and plates with phase-switchable photosensitivelayer). Preferably, the photosensitive layer exhibits an affinity oraversion substantially opposite to the affinity or aversion of thesubstrate to at least one printing liquid selected from the groupconsisting of ink and an adhesive fluid for ink (as for plates withnon-phase-switchable photosensitive layer, which can be wet plate orwaterless plate). More preferably, the plate has a hydrophilic substrateand an oleophilic photosensitive layer (as for wet plate withnon-phase-switchable photosensitive layer). An adhesive fluid for ink isa fluid that repels ink. Fountain solution is the most commonly usedadhesive fluid for ink. A wet plate is printed on a wet press equippedwith both ink and fountain solution, while a waterless plate is printedon a waterless press equipped with ink.

Usually, as for most printing plates described in the literature, thephotosensitive layer exhibits an affinity or aversion substantiallyopposite to the affinity or aversion of the substrate to at least oneprinting liquid selected from the group consisting of ink and anadhesive fluid for ink, and does not switch its affinity or aversionupon laser exposure. However, certain photosensitive layer exhibitssubstantially the same affinity or aversion as the substrate and iscapable of switching to opposite affinity or aversion upon exposure to alaser (with or without further treatment such as on-press developmentwith ink and/or fountain solution), as described in U.S. Pat. Nos.6,331,375, 5,910,395, 6,720,464, and 6,136,503. Bothnon-phase-switchable photosensitive layer and phase-switchablephotosensitive layer can be used for the current invention. Preferred isa non-phase-switchable photosensitive layer (coated on a substrate withopposite affinity or aversion). More preferred is an oleophilicphotosensitive layer (coated on a hydrophilic substrate).

In this patent, the term color change or the term discoloration meansany change on the appearance of the color, such as changing to adifferent color (such as from blue to green), increasing in colorstrength (such as becoming bluer), decreasing in color strength (such asbecoming less blue), turning off of a color (such as with a blue colorcompletely disappearing), or formation (turning on) of a color (such asfrom colorless to blue). The term yellow or red light means yellowlight, red light, or any light with a color between yellow and red suchas orange light. The term safe light means a light with a certainwavelength range being cut off, including a yellow light or red light,so that it does not cause hardening of a certain photosensitive layer.The term white light means a white fluorescent light, a whiteincandescent light, sunlight, or any white office light. The term whiteroom light means a typical white office light (with white fluorescentlight). The term substantially no radiation below a wavelength means theintensity of the radiation below that wavelength is less than 1% of thatfor a regular 100-watt incandescent light (for home use, not focused) ata distance of 2 meters. The term substantial darkness means theintensity of the radiation is less than 1% of that for a regular100-watt incandescent light at a distance of 2 meters. The termsubstantially light-tight means less than 1% of the room light can passthrough. The term substantially all means at least 99%. The term “withthe plate under a room light” means the plate is exposed to such roomlight; i.e., such room light reaches the plate. The term monomerincludes both polymerizable monomer and polymerizable oligomer. The term(meth)acrylate includes acrylate and/or methacrylate (acrylate,methacrylate, or both acrylate and methacrylate).

For preparing lithographic printing plates of the current invention, anyphotosensitive layer is suitable which is soluble or dispersible in inkand/or fountain solution, and is capable of hardening upon exposure to alaser having a wavelength selected from 200 to 1200 nm. Here hardeningmeans becoming insoluble and non-dispersible in ink and/or fountainsolution. Hardening is generally achieved through crosslinking orpolymerization of the resins (polymers or monomers). A laser sensitivedye or pigment (preferably a sensitizing dye) is preferably added in thephotosensitive layer. The photosensitive layer preferably has a coverageof from 100 to 3000 mg/m², more preferably from 200 to 2000 mg/m², andmost preferably from 400 to 1500 mg/m².

Preferably, the photosensitive layer comprises a polymerizable monomerand an initiating system, optionally with addition of a polymericbinder. The initiating system generally comprises an initiator; aninitiator and a sensitizing dye; or an initiator, a sensitizing dye anda hydrogen donor; depending on the specific photosensitive layer. Eitherone species (such as 1 initiator or 1 polymer) or more than one speciesof the same component type (such as 2 different initiators or 3different monomers) can be added in the same photosensitive layer. Thecomposition ratios (such as monomer to polymer ratio) are usuallydifferent from conventional plates designed for development with aregular liquid developer. Various additives may be added to, forexample, allow or enhance on-press developability. Such additivesinclude surfactant, plasticizer, water soluble polymer or smallmolecule, and ink soluble polymer or small molecule. The addition ofnonionic surfactant is especially helpful in making the photosensitivelayer dispersible with ink and fountain solution, or emulsion of ink andfountain solution. Various additives useful for conventionalphotosensitive layer can also be used. These additives include pigment,dye, exposure indicator, and stabilizer.

Photosensitive materials useful in wet plates of this invention include,for example, photosensitive compositions comprising a polymerizablemonomer, an initiator, a sensitizing dye, and optionally a polymericbinder.

Photosensitive oleophobic materials useful in waterless plates of thisinvention include, for example, compositions comprising a monomer havingperfluoroalkyl or polysiloxane groups and crosslinkable terminal groups,an initiator, and a sensitizing dye.

Infrared laser sensitive (also called thermosensitive) materials usefulfor wet lithographic plates of this invention include, for example,thermosensitive compositions comprising a polymerizable monomer, aninitiator, an infrared absorbing dye, and optionally a polymeric binder.

Visible or ultraviolet light sensitive materials useful for wet platesof this invention include, for example, photosensitive compositionscomprising a polymerizable monomer, an initiator, a visible orultraviolet light sensitizing dye, and optionally a polymeric binder. Ahydrogen donor is preferably added to accelerate the polymerization.

Polymeric binder for the photosensitive layer of this invention can beany solid film-forming polymer. Such polymer may or may not have(meth)acrylate groups or other ethylenic groups (such as allyl groups).Examples of suitable polymeric binders include (meth)acrylic polymersand copolymers (such as polybutylmethacrylate, polyethylmethacrylate,polymethylmethacrylate, polymethylacrylate,butylmethacrylate/methylmethacrylate copolymer,methylmethacrylate/methylmethacrylic acid copolymer,polyallylmethacrylate, and allylmethacrylate/methacrylic acidcopolymer), polyvinyl acetate, polyvinyl butyrate, polyvinyl chloride,styrene/acrylonitrile copolymer, styrene/maleic anhydride copolymer andits partial ester, nitrocellulose, cellulose acetate butyrate, celluloseacetate propionate, vinyl chloride/vinyl acetate copolymer,butadiene/acrylonitrile copolymer, polyurethane binder, and polymericbinder having polymer backbone with recurring units having pendantpoly(alkylene glycol) side chains. The polymeric binder suitable for thephotosensitive layer of this invention has a weight average molecularweight of at least 5,000, preferably from 10,000 to 1,000,000, morepreferably from 20,000 to 500,000, and most preferably from 50,000 to200,000 Dalton. It is noted that polymeric compounds with weight averagemolecular weight of less that 5,000 can also be added in thephotosensitive layer of this invention; however, in order to avoidconfusion, such compounds are not considered as polymeric binder and arecalled oligomer (without or with polymerizable groups) in thisapplication (oligomers having polymerizable groups are also included inthe definition of monomers in this application).

Suitable free-radical polymerizable monomers include any monomer oroligomer with at least one ethylenically unsaturated group. Suchmonomers include monofunctional, difunctional, and multifunctional(meth)acrylate monomers or oligomers, such as (meth)acrylate esters ofethylene glycol, trimethylolpropane, pentaerythritol, ethoxylatedethylene glycol and ethoxylated trimethylolpropane; multifunctionalurethanated (meth)acrylate; epoxylated (meth)acrylate; oligomeric amine(meth)acrylate; and phosphate ester-containing (meth)acrylate (such asphosphate ester of 2-hydroxyethyl methacrylate, and various phosphateester containing (meth)acrylate monomers as described in U.S. Pat. Nos.4,101,326, 5,679,485, 5,776,655 and 7,316,887, and U.S. Pat. App. No.2008/0008957). The monomers can be urethane (meth)acrylate, ornon-urethane (meth)acrylate. Combination of both urethane (meth)acrylateand non-urethane (meth)acrylate monomers can be used. The monomerspreferably has at least 3 (meth)acrylate groups, more preferably atleast 4 (meth)acrylate groups, even more preferably at least 5(meth)acrylate groups, and most preferably at least 6 (meth)acrylategroups. However, monofunctional or difunctional (meth)acrylate monomercan be added into the photosensitive layer having multifunctional(meth)acrylate monomers; the total amount of such monofunctional ordifunctional monomers is preferably less than 50% by weight of the totalmonomers, more preferably less than 30%, and most preferably less than10%. Acrylate monomer is preferred over methacrylate monomer because ofthe faster photospeed of acrylate group over methacrylate group. Themonomer has a molecular weight of less than 5,000, preferably from 100to 3,000, more preferably from 200 to 2,000, and most preferably from300 to 1,500 Dalton.

Urethane (meth)acrylate monomers include any compounds having at leastone urethane linkage (—NHCOO—) and at least one (meth)acrylate group.Preferred urethane (metha)acrylate monomers are those with at least 3(meth)acrylate groups, more preferably at least 4 (meth)acrylate groups,even more preferably at least 5 (meth)acrylate groups, and mostpreferably at least 6 (meth)acrylate groups. Urethane (meth)acrylatemonomer is usually formed by reacting a compound having at least oneisocyanate group with a (meth)acrylate compound having a hydroxy group.Urethane monomer with 2 or more (meth)acrylate groups are usually formedfrom a compound having one or more isocyanate groups and a(meth)acrylate compound having a hydroxy group and one or more(meth)acrylate groups. For example, a tetrafunctional urethane(meth)acrylate monomer can be formed from a compound having one hydroxygroup and 2 (meth)acrylate groups with a bifunctional isocyanatecompound. Suitable isocyanate compounds include, for example, aromaticdiisocyanate such as p-phenylene diisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylene diisocyanate, 4,4′-diphenylmethanediisocyanate, naphthalene-1,5-diisocyanate and tolydine diisocyanate;aliphatic diisocyanate such as hexamethylene diisocyanate, lysinemethylester diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate and dimeracid diisocyanate; alicyclic diisocyanate such as isophoronediisocyanate, and 4,4′-methylenebis(cyclohexylisocyanate); aliphaticdiisocyanate having an aromatic ring, such as xylylene diisocyanate;triisocyanate such as lysine ester triisocyanate, 1,6,11-undecanetriisocyanate, 1,8-diisocyanate-4-isocyanatemethyloctane,1,3,6-hexamethylene triisocyanate, bicycloheptane triisocyanate,tris(isocyanate phenylmethane) and tris(isocyanatephenyl)thiophosphate;and polyisocyanate formed from condensation of three or morediisocyanate compounds such as 2,4-tolylene diisocyanate isocyanuratetrimer, 2,4-tolylene diisocyanate-trimethylolpropane adduct,1,6-hexanediisocyante isocyanurate trimer. Suitable (meth)acrylatecompounds with one hydroxy group include pentaerythritoltri(meth)acrylate, dipentaerythritol penta(meth)acrylate,ditrimethylolpropane tri(meth)acrylate and pentaerythritoldi(meth)acrylate monostearate. Various urethane (meth)acrylate monomersare described in U.S. Pat. No. 6,232,038 and U.S. Pat. Pub. No.2002/0018962, and can be used as the urethane (meth)acrylate monomers ofthis instant invention. Among the urethane (meth)acrylate monomers,urethane acrylate monomer is preferred. Either aromatic urethane(meth)acrylate monomer (which contains at least one aromatic group inthe molecule) or aliphatic urethane (meth)acrylate monomer (which doesnot contain any aromatic group in the molecule) or both can be used in aphotosensitive layer of this invention.

Suitable non-urethane (meth)acrylate monomers can be any (meth)acrylatemonomers without urethane linkage (—NHCOO—) in the molecule. Suitablenon-urethane (meth)acrylate monomers include, for example,trimethylolpropane triacrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol penta(meth)acrylate, dipentaerythritolhexa(meth)acrylate, di(trimethylolpropane) tetra(meth)acrylate. Amongthe non-urethane (meth)acrylate monomers, non-urethane acrylate monomeris preferred.

Suitable free-radical initiators include, for example, the derivativesof acetophenone (such as 2,2-dimethoxy-2-phenylacetophenone and2-methyl-1-[4-(methylthio)phenyl]-2-morpholino propan-1-one), oniumsalts such as diaryliodonium hexafluoroantimonate, diaryliodoniumhexafluorophosphate, diaryliodonium triflate,(4-(2-hydroxytetradecyl-oxy)phenyl)phenyliodonium hexafluoroantimonate,(4-octoxyphenyl)phenyliodonium hexafluoroantimonate,bis(4-t-butylphenyl)iodonium hexafluorophosphate, triarylsulfoniumhexafluorophosphate, triarylsulfonium p-toluenesulfonate,(3-phenylpropan-2-onyl)triaryl phosphonium hexafluoroantimonate andN-ethoxy(2-methyl)pyridinium hexafluorophosphate, and the onium salts asdescribed in U.S. Pat. Nos. 5,955,238, 6,037,098 and 5,629,354; boratesalts such as tetrabutylammonium triphenyl(n-butyl)borate,tetraethylammonium triphenyl(n-butyl)borate, diphenyliodoniumtetraphenylborate, and triphenylsulfonium triphenyl(n-butyl)borate, andthe borate salts as described in U.S. Pat. Nos. 6,232,038 and 6,218,076;haloalkyl substituted s-triazines such as2,4-bis(trichloromethyl)-6-(p-methoxy-styryl)-s-triazine,2,4-bis(trichloromethyl)-6-(4-methoxy-naphth-1-yl)-s-triazine,2,4-bis(trichloromethyl)-6-piperonyl-s-triazine, and2,4-bis(trichloromethyl)-6-[(4-ethoxyethylenoxy)-phen-1-yl]-s-triazine,and the s-triazines as described in U.S. Pat. Nos. 5,955,238, 6,037,098,6,010,824, and 5,629,354; titanocene compounds such asbis(η⁹-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl)titanium; hexaarylbiimidazolecompounds such as2,2′-bis(2-chlorophenyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole,2,2′-bis(2-ethoxyphenyl)-4,4′,5,5′-tetraphenyl-1,1′-biimidazole,2-(1-naphthyl)-4,5-diphenyl-1,2′-biimidazole; and derivatives ofacetophenone such as 2,2-dimethoxy-2-phenylacetophenone, and2-methyl-1-[4-(methylthio)phenyl]-2-morpholino propan-1-one.Triarylsulfonium salts, diaryliodonium salts, and triarylalkylboratesalts are preferred initiators for infrared laser sensitive plate.Titanocene compounds and hexaarylbiimidazole compounds are preferredinitiators for visible or ultraviolet laser sensitive plate, andhexaarylbiimidazole compounds are more preferred. The initiator is addedin the photosensitive layer preferably at 0.1 to 40% by weight of thephotosensitive layer, more preferably 1 to 30%, and most preferably 5 to20%.

Suitable sensitizing dyes in this invention include any compoundscapable of absorbing an imaging radiation and transferring the absorbedradiation energy to the initiator or other component in thephotosensitive layer to cause hardening of the photosensitive layer.Suitable sensitizing dyes include infrared sensitizing dyes (also calledinfrared absorbing dyes), visible sensitizing dyes (including violetsensitizing dyes), and ultraviolet sensitizing dyes. Preferred areinfrared absorbing dyes and violet or ultraviolet sensitizing dyes. Morepreferred are infrared laser absorbing dyes and violet or ultravioletlaser sensitizing dyes.

Infrared sensitizing dyes useful in the thermosensitive layer of thisinvention include any infrared absorbing dye effectively absorbing aninfrared radiation having a wavelength of 750 to 1200 nm. It ispreferable that the dye has an absorption maximum between thewavelengths of 780 and 1100 nm. Various infrared absorbing dyes aredescribed in U.S. Pat. Nos. 5,858,604, 5,922,502, 6,022,668, 5,705,309,6,017,677, and 5,677,106, and in the book entitled “Infrared AbsorbingDyes” edited by Masaru Matsuoka, Plenum Press, New York (1990), and canbe used in the thermosensitive layer of this invention. Examples ofuseful infrared absorbing dyes include squarylium, croconate, cyanine(including polymethine), phthalocyanine (including naphthalocyanine),merocyanine, chalcogenopyryloarylidene, oxyindolizine, quinoid,indolizine, pyrylium and metal dithiolene dyes. Cyanine andphthalocyanine dyes are preferred infrared absorbing dyes. The infraredlaser sensitizing dye is added in the photosensitive layer preferably at0.1 to 20% by weight of the photosensitive layer, more preferably 0.5 to10%, and most preferably 1 to 5%.

Visible or ultraviolet sensitizing dyes useful in the visible orultraviolet sensitive photosensitive layer of this invention include anydyes having a wavelength maximum of from 200 to 600 nm. Suitable visibleor ultraviolet sensitizing dyes include, for example, cyanine dyes;rhodamine compounds such as rhodamine 6G perchloride; chromanonecompounds such as 4-diethylaminobenzilidene chromanone;dialkylaminobenzene compounds such as ethyl 4-dimethylaminobenzoate anddialkylaminobenzene; dialkylaminobenzophenone compounds such as4,4′-bis(dimethylamino)benzophenone, 4,4′-bis(diethylamino)benzophenone,2-(p-dimethylaminophenyl)benzooxazole,2-(p-diethylaminophenyl)benzooxazole,2-(p-dimethylaminophenyl)benzo[4,5]benzooxazole,2-(p-dimethylaminophenyl)benzo[6,7]benzooxazole,2,5-bis(p-diethylaminophenyl)1,3,4-oxazole,2-(p-dimethylaminophenyl)benzothiazole,2-(p-diethylaminophenyl)benzothiazole,2-(p-dimethylaminophenyl)benzimidazole,2-(p-diethylaminophenyl)benzimidazole,2,5-bis(p-diethylaminophenyl)1,3,4-thiadiazole,(p-dimethylaminophenyl)pyridine, (p-diethylaminophenyl)pyridine,2-(p-dimethylaminophenyl)quinoline, 2-(p-diethylaminophenyl)quinoline,2-(p-dimethylaminophenyl)pyrimidine or2-(p-diethylaminophenyl)pyrimidine; unsaturated cyclopentanone compoundssuch as2,5-bis{[4-(diethylamino)phenyl]methylene}-(2E,5E)-(9Cl)-cyclopentanoneand bis(methylindolenyl)cyclopentanone; coumarin compounds such as3-benzoyl-7-methoxy coumarin and 7-methoxy coumarin; and thioxanthenecompounds such as 2-isopropylthioxanthenone. Dialkylaminobenzenecompounds and bis(dialkylamino)benzophenone compounds are particularlysuitable for ultraviolet laser sensitive plate.Bis(dialkylamino)benzophenone compounds are particularly suitable forviolet laser sensitive plate. The sensitizing dyes as described in U.S.Pat. Nos. 5,422,204 and 6,689,537, and U.S. Pat. App. Pub. No.2003/0186165 can be used for the photosensitive layer of this invention.The visible or ultraviolet laser sensitizing dye is added in thephotosensitive layer preferably at 0.1 to 20% by weight of thephotosensitive layer, more preferably 0.5 to 15%, and most preferably 1to 10%.

The free radical polymerizable photosensitive composition of the presentinvention can contain one or more hydrogen donors as polymerizationaccelerator. Examples of the hydrogen donors include compounds having amercapto group (also called mercapto compounds) such as2-mercaptobenzoxazole, 2-mercaptobenzothiazole, 2-mercaptobenzimidazoleand 3-mercapto-1,2,4-triazole; and N-aryl-α-amino acids, their salts andesters such as N-phenylglycine, salts of N-phenylglycine, and alkylesters of N-phenylglycine such as N-phenylglycine ethyl ester andN-phenylglycine benzyl ester. Preferred hydrogen donors are mercaptocompounds and N-aryl-α-amino acids, their salts and esters; morepreferred are mercapto compounds. Combination of at least one mercaptocompound and at least one N-aryl-α-amino acid or its ester or salt canbe advantageously used in the photosensitive layer. The hydrogen donoris preferably added in the photosensitive layer at 0.01 to 15% by weightof the photosensitive layer, more preferably 0.1 to 10%, and mostpreferably 0.5 to 5%.

Various surfactants can be added into the photosensitive layer to allowor enhance the on-press developability with ink and/or fountain. Bothpolymeric and small molecule surfactants can be used. However, it ispreferred that the surfactant has low or no volatility so that it willnot evaporate from the photosensitive layer of the plate during storageand handling. Nonionic surfactants are preferred. The nonionicsurfactant used in this invention should have sufficient portion ofhydrophilic segments (or groups) and sufficient portion of oleophilicsegments (or groups), so that it is at least partially soluble in water(>1 g surfactant soluble in 100 g water) and at least partially solublein organic phase (>1 g surfactant soluble in 100 g photosensitivelayer). Preferred nonionic surfactants are polymers and oligomerscontaining one or more polyether (such as polyethylene glycol,polypropylene glycol, and copolymer of ethylene glycol and propyleneglycol) segments. Examples of preferred nonionic surfactants are blockcopolymers of propylene glycol and ethylene glycol (also called blockcopolymer of propylene oxide and ethylene oxide); ethoxylated orpropoxylated acrylate oligomers; and polyethoxylated alkylphenols andpolyethoxylated fatty alcohols. The nonionic surfactant is preferablyadded at from 0.1 to 30% by weight of the photosensitive layer, morepreferably from 0.5 to 20%, and most preferably from 1 to 15%.

A hydrophilic or oleophilic micro particles may be added into thephotosensitive layer to enhance, for example, the on-pressdevelopability and non-tackiness. Suitable micro particles includepolymer particles, talc, titanium dioxide, barium sulfate, siliconeoxide, and aluminum micro particles, with an average particle size ofless than 10 microns, preferably less than 5 microns, more preferablyless than 2 microns, and most preferably less than 1 micron. A suitableparticular dispersion is described in U.S. Pat. No. 6,071,675, theentire disclosure of which is hereby incorporated by reference.

For plates with rough and/or porous surface capable of mechanicalinterlocking with a coating deposited thereon, a thin water-solubleinterlayer may be deposited between the substrate and the photosensitivelayer. Here the substrate surface is rough and/or porous enough and theinterlayer is thin enough to allow bonding between the photosensitivelayer and the substrate through mechanical interlocking. Such a plateconfiguration is described in U.S. Pat. No. 6,014,929, the entiredisclosure of which is hereby incorporated by reference. Preferredreleasable interlayer comprises a water-soluble polymer. Polyvinylalcohol (including various water-soluble derivatives of polyvinylalcohol) is the preferred water-soluble polymer. Usually purewater-soluble polymer is coated. However, one or more surfactant andother additives may be added. The water-soluble polymer is generallycoated from an aqueous solution with water as the only solvent. Awater-soluble organic solvent, preferably an alcohol such as ethanol orisopropanol, can be added into the water-soluble polymer aqueous coatingsolution to improve the coatability. The water-soluble organic solventis preferably added at less than 20% by weight of the solution, morepreferably at less than 10%. The releasable interlayer preferably has anaverage coverage of 1 to 200 mg/m², more preferably 2 to 100 mg/m², andmost preferably 4 to 40 mg/m². The substrate preferably has an averagesurface roughness Ra of 0.2 to 2.0 microns, and more preferably 0.4 to1.0 microns.

The photosensitive layer may be conformally coated onto a roughenedsubstrate (for example, with Ra of larger than 0.4 microns) at thincoverage (for example, of less than 1.2 g/m²) so that the plate can havemicroscopic peaks and valleys on the photosensitive layer coatedsurface, and exhibit low tackiness and good block resistance (beforecoating overcoat during manufacture or after removing overcoat beforemounting on press), as described in U.S. Pat. No. 6,242,156, the entiredisclosure of which is hereby incorporated by reference.

In a preferred embodiment for the thermosensitive lithographic printingplate of this invention, the thermosensitive layer comprises a polymericbinder, a polymerizable monomer having at least one terminal ethylenicgroup, a free-radical initiator, and an infrared absorbing dye. Otheradditives such as surfactant, dye or pigment, exposure-indicating dye(such as leuco crystal violet, leucomalachite green, azobenzene,4-phenylazodiphenylamine, and methylene blue dyes), and free-radicalstabilizer (such as methoxyhydroquinone) may be added. The weight ratioof all the monomers to all the polymeric binders is preferably at least1.0, more preferably from 1.5 to 6.0, and most preferably from 2.0 to5.0.

In another preferred embodiment for the thermosensitive lithographicplates of this invention, the thermosensitive layer comprises apolymeric binder, a urethane (meth)acrylate monomer having at least 4(meth)acrylate groups, a free-radical initiator, and an infraredabsorbing dye. A mercapto group-containing compound is preferably added.The weight ratio of all the monomers to all the polymeric binders ispreferably at least 1.0, more preferably from 1.5 to 6.0, and mostpreferably from 2.0 to 5.0.

In yet another preferred embodiment for the thermosensitive lithographicplates of this invention, the thermosensitive layer comprises apolymeric binder having polymer backbone with recurring units havingpendant poly(alkylene glycol) side chains, a (meth)acrylate monomerhaving at least one (meth)acrylate group, a free-radical initiator, andan infrared absorbing dye. A mercapto group-containing compound ispreferably added. The weight ratio of all the monomers to all thepolymeric binders is preferably at least 1.0, more preferably from 1.5to 6.0, and most preferably from 2.0 to 5.0.

In further another preferred embodiment for the thermosensitivelithographic plates of this invention, the thermosensitive layercomprises a polymeric binder, a urethane (meth)acrylate monomer havingat least 4 (meth)acrylate groups, a phosphate ester-containing(meth)acrylate monomer, a free-radical initiator, and an infraredabsorbing dye. A mercapto group-containing compound is preferably added.The weight ratio of all the monomers to all the polymeric binders ispreferably at least 1.0, more preferably from 1.5 to 6.0, and mostpreferably from 2.0 to 5.0.

In yet further another preferred embodiment for the thermosensitivelithographic plates of this invention, the thermosensitive layercomprises a polymeric binder, a urethane (meth)acrylate monomer havingat least 4 (meth)acrylate groups, a non-urethane (meth)acrylate monomerhaving at least 4 (meth)acrylate groups, a free-radical initiator, andan infrared absorbing dye. The weight ratio of all the monomers to allthe polymeric binders is preferably at least 1.0, more preferably from1.5 to 6.0, and most preferably from 2.0 to 5.0.

In a preferred embodiment for visible light sensitive lithographicprinting plates of this invention, the photosensitive layer comprises apolymeric binder, a polymerizable monomer having at least one terminalethylenic group, a free-radical initiator, and a visible sensitizingdye. A hydrogen donor is preferably added. Other additives such assurfactant, dye or pigment, exposure-indicating dye, and free-radicalstabilizer may be added. The weight ratio of all the monomers to all thepolymeric binders is preferably at least 1.0, more preferably from 1.5to 6.0, and most preferably from 2.0 to 5.0.

In a preferred embodiment for violet or ultraviolet light sensitivelithographic printing plates of this invention, the photosensitive layercomprises a polymeric binder, a polymerizable monomer having at leastone terminal ethylenic group, a free-radical initiator, and a violet orultraviolet sensitizing dye. A hydrogen donor is preferably added. Otheradditives such as surfactant, dye or pigment, exposure-indicating dye,and free-radical stabilizer may be added. The weight ratio of all themonomers to all the polymeric binders is preferably at least 1.0, morepreferably from 1.5 to 6.0, and most preferably from 2.0 to 5.0.

In another preferred embodiment for the violet or ultraviolet lasersensitive lithographic plates of this invention, the photosensitivelayer comprises a polymeric binder, a monomer having at least 3(meth)acrylate group, a hexaarylbiimidazole or titanocene compound, anda dialkylaminobenzophenone compound. A mercapto group-containingcompound is preferably added. The weight ratio of all the monomers toall the polymeric binders is preferably at least 1.0, more preferablyfrom 1.5 to 6.0, and most preferably from 2.0 to 5.0. Ahexaarylbiimidazole compound is preferred among hexaarylbiimidazole andtitanocene compounds. A preferred dialkylaminobenzophenone compound is a4,4′-bis(dialkylamino)benzophenone compound.

In yet another preferred embodiment for the violet or ultraviolet lasersensitive lithographic plates of this invention, the photosensitivelayer comprises a polymeric binder, a urethane monomer having at least 4(meth)acrylate groups, a hexaarylbiimidazole or titanocene compound, anda dialkylaminobenzophenone compound. A mercapto group-containingcompound is preferably added. The weight ratio of all the monomers toall the polymeric binders is preferably at least 0.5, more preferablyfrom 1.0 to 6.0, and most preferably from 2.0 to 5.0. Ahexaarylbiimidazole compound is preferred among hexaarylbiimidazole andtitanocene compounds. A preferred dialkylaminobenzophenone compound is a4,4′-bis(dialkylamino)benzophenone compound. A non-urethane(meth)acrylate monomer is preferably added.

In further another preferred embodiment for the violet or ultravioletlaser sensitive lithographic plates of this invention, thephotosensitive layer comprises a polymeric binder, a urethane monomerhaving at least 4 (meth)acrylate groups, a non-urethane monomer havingat least 4 (meth)acrylate groups, a free radical initiator, and a violetor ultraviolet sensitizing dye. A mercapto group-containing compound ispreferably added. The weight ratio of the urethane (meth)acrylatemonomer to the non-urethane (meth)acrylate monomer is preferably from0.10 to 10.0, more preferably from 0.20 to 5.0, and most preferably from0.30 to 3.0. The weight ratio of all the monomers to all the polymericbinders is preferably at least 0.5, more preferably from 1.0 to 6.0,even more preferably from 1.5 to 5.0, and most preferably from 2.0 to4.0. A preferred free radical initiator is a hexaarylbiimidazole ortitanocene compound, more preferably a hexaarylbiimidazole compound. Apreferred sensitizing dye is a dialkylaminobenzophenone compound, morepreferably a 4,4′-bis(dialkylamino)benzophenone compound.

In yet further another preferred embodiment for the violet orultraviolet laser sensitive lithographic plates of this invention, thephotosensitive layer comprises a polymeric binder having polymerbackbone with recurring units having pendant poly(alkylene glycol) sidechains, a (meth)acrylate monomer having at least one (meth)acrylategroup, a free-radical initiator, and a violet or ultraviolet sensitizingdye. A mercapto group-containing compound is preferably added. Otheradditives such as surfactant, dye or pigment, exposure-indicating dye,and free-radical stabilizer may be added. The weight ratio of all themonomers to all the polymeric binders is preferably at least 1.0, morepreferably from 1.5 to 6.0, and most preferably from 2.0 to 5.0.

In also further another preferred embodiment for the violet orultraviolet laser sensitive lithographic plates of this invention, thephotosensitive layer comprises a polymeric binder, a urethane monomerhaving at least 4 (meth)acrylate groups, a phosphate ester-containing(meth)acrylate monomer, a free radical initiator, and a violet orultraviolet sensitizing dye. A mercapto group-containing compound ispreferably added. The weight ratio of all the monomers to all thepolymeric binders is preferably at least 0.5, more preferably from 1.0to 6.0, and most preferably from 2.0 to 5.0. A preferred free radicalinitiator is a hexaarylbiimidazole or titanocene compound, morepreferably a hexaarylbiimidazole compound. A preferred sensitizing dyeis a dialkylaminobenzophenone compound, more preferably a4,4′-bis(dialkylamino)benzophenone compound. A phosphate-freenon-urethane (meth)acrylate monomer can be added.

For preparing lithographic printing plate capable of discolorationselectively in the non-hardened area with a discoloring solution(preferably an aqueous rinsing solution containing a discoloring agent),the above plates preferably further comprise a visible dye, a dispersedpigment, or a latent dye in the photosensitive layer.

The overcoated on-press developable lithographic plates as described inU.S. Pat. Nos. 6,482,571, 6,576,401, 5,548,222 and 6,541,183, and U.S.patent application Ser. Nos. 10/720,882, 11/075,663, 11/175,518,11/266,817, 11/356,911, 11/728,648, 11/787,878, 11/800,634, 11/810,710,11/825,576, 11/859,756, 11/944,204, 11/967,961, and 12/030,867, theentire disclosures of which are hereby incorporated by reference, can beused for the instant invention.

The laser for the imagewise exposure in this application can be anylaser having a wavelength selected from 200 to 1200 nm which is capableof causing hardening to the photosensitive layer, preferably a violet orultraviolet laser of from 200 to 430 nm or an infrared laser of 750 to1200 nm.

Infrared lasers useful for the imagewise exposure of the thermosensitiveplates of this invention include laser sources emitting in the nearinfrared region, i.e. emitting in the wavelength range of from 750 to1200 nm, and preferably from 800 to 1100 nm. Particularly preferredinfrared laser sources are laser diode emitting around 830 nm or NdYAGlaser emitting around 1060 nm. The plate is exposed at a laser dosagethat is sufficient to cause hardening in the exposed areas but not highenough to cause substantial thermal ablation. The exposure dosage ispreferably from 1 to 500 mJ/cm², more preferably from 5 to 200 mJ/cm²,and most preferably from 20 to 150 mJ/cm², depending on the sensitivityof the thermosensitive layer.

Visible lasers useful for the imagewise exposure of the visible lightsensitive plates of this invention include any laser emitting in thewavelength range of from 390 to 600 nm. Examples of suitable visiblelasers include frequency-doubled Nd/YAG laser (about 532 nm), argon ionlaser (about 488 nm), violet diode laser (about 390 to 430 nm), andvisible LEDs. Violet laser diode is especially useful because of itssmall size and relatively low cost. The exposure dosage is preferablyfrom 0.0001 to 5 mJ/cm² (0.1 to 5000 μJ/cm²), more preferably from 0.001to 0.5 mJ/cm² (1 to about 500 μJ/cm²), and most preferably from 0.005 to0.10 mJ/cm² (5 to 100 μJ/cm²), depending on the sensitivity of thephotosensitive layer.

Ultraviolet lasers useful for the imagewise exposure of the ultravioletlight sensitive plates of this invention include any laser having awavelength of from 200 to 390 nm. Examples of ultraviolet lasers includeultraviolet diode lasers or LEDs having a wavelength of from 350 to 390nm. Laser diodes are preferred ultraviolet lasers. The exposure dosageis preferably from 0.0001 to 5 mJ/cm² (0.1 to 5000 μJ/cm²), morepreferably from 0.001 to 0.5 mJ/cm² (1 to about 500 μJ/cm²), and mostpreferably from 0.005 to 0.10 mJ/cm² (5 to 100 μJ/cm²), depending on thesensitivity of the photosensitive layer.

Among the visible and ultraviolet lasers, particularly useful is violetor ultraviolet laser with a wavelength selected from 200 to 430 nm,preferably from 300 to 430 nm.

Laser imaging devices are currently widely available commercially. Anydevice can be used which provides laser exposure to the plate accordingto digital imaging information. Commonly used imaging devices includeflatbed imager, internal drum imager, and external drum imager, all ofwhich can be used for the imagewise laser exposure in this invention.

The plate can be exposed with a laser on an laser imager, stripped offthe overcoat (preferably on a overcoat removing device, which can bestandalone or can be connected to or part of the laser imager,preferably connected to or part of the laser imager), and then mountedon press to develop with ink and/or fountain solution and then print outregular printed sheets. The ink and/or fountain solution solubilized ordispersed photosensitive layer and/or overcoat can be mixed into the inkand/or the fountain solution on the rollers, and/or can be transferredto the blanket and then the receiving medium (such as paper). Thefountain solution roller is engaged (to the plate cylinder as forconventional inking system or to the ink roller as for integrated inkingsystem) for preferably 0 to 100 rotations, more preferably 1 to 50rotations and most preferably 5 to 20 rotations (of the plate cylinder),and the ink roller is then engaged to the plate cylinder for preferably0 to 100 rotations, more preferably 1 to 50 rotations and mostpreferably 5 to 20 rotations before engaging the plate cylinder andfeeding the receiving medium. Good quality prints should be obtainedpreferably under 40 initial impressions, more preferably under 20impressions, and most preferably under 5 impressions.

It is noted that for a good quality print, the imaged areas should havegood inking and the background areas should be free of ink (with cleanbackground). Any background toning is not desirable.

The term “background toning” means that the non-imaged areas (withphotosensitive layer removed from the substrate during development) ofthe developed plate, as well as the non-imaged areas of the printedsheets, accept small amount of ink and are not completely clean, so thatthe non-imaged areas of a printed paper shows slightly different color(such as slightly gray for a white paper with background toning whenprinted with black ink, or slightly pink for a white paper withbackground toning when printed with red ink). Background toning isusually caused by partial curing of the photosensitive layer, incompleteremoval of the photosensitive layer during development, insufficienthydrophilicity of the substrate, or insufficient fountain solution levelof the press. A plate of this invention which is originally free ofbackground toning can become having background toning after exposure toa white room light for a certain amount of time due to partial curing ofthe photosensitive layer; further exposure to such white room light may,or may not, lead to full hardening of the photosensitive layer,depending on the particular plate. In normal printing operation as wellas in this application, sufficient fountain solution level on pressshould be maintained, so that the background toning should not be causedby insufficient fountain solution level.

It is noted that some negative plate will become having backgroundtoning after exposure to white room light for a certain amount of timeand become filly hardened after further exposure to white room light,some negative plate will become having background toning after exposureto white room light for a certain amount of time but will never becomefully hardened with further exposure to white room light, some negativeplate will become fully hardened under white room light quickly (such asunder 10 minutes), and some negative plate will never become fullyhardened or having background toning even after exposure to white roomlight for a long period of time (such as at least 24 hours). Here theterm “fully hardened” means that the photosensitive layer in specificareas is sufficiently cured so that substantially the wholephotosensitive layer in such areas remains on the plate afterdevelopment. The plates suitable for the instant invention are capableof hardening or background toning under a white room light for a limitedtime period (preferably less than a time period selected from 1 to 60minutes) and are incapable of hardening or causing background toningunder said while room light for at least twice of said time period.

During on-press development, ink and fountain solution may be applied tothe plate on a lithographic press at any combination or sequence, asneeded for the plate. For conventional wet press, preferably fountainsolution roller is contacted to the plate first, followed by contactingwith ink roller. For press with integrated inking/dampening system,preferably the ink and fountain solution are emulsified by various pressrollers before being transferred to the plate as emulsion of ink andfountain solution.

The ink used in this application can be any ink suitable forlithographic printing. Most commonly used lithographic inks include “oilbased ink” which crosslinks upon exposure to the oxygen in the air and“rubber based ink” which does not crosslink upon exposure to the air.Specialty inks include, for example, radiation-curable ink and thermallycurable ink. An ink is an oleophilic, liquid or viscous material whichgenerally comprises a pigment dispersed in a vehicle, such as vegetableoils, animal oils, mineral oils, and synthetic resins. Variousadditives, such as plasticizer, surfactant, drier, drying retarder,crosslinker, and solvent may be added to achieve certain desiredperformance. The compositions of typical lithographic inks are describedin “The Manual of Lithography” by Vicary, Charles Scribner's Sons, NewYork, and Chapter 8 of “The Radiation Curing: Science and Technology” byPappas, Plenum Press, New York, 1992.

The fountain solution used in this application can be any fountainsolution used in lithographic printing. Fountain solution is used in wetlithographic printing press to dampen the hydrophilic areas (non-imageareas), repelling ink (which is hydrophobic and oleophilic) from theseareas. Fountain solution contains mainly water, generally with additionof certain additives such as gum arabic and surfactant. Small amount ofalcohol such as isopropanol can also be added in the fountain solution.Water is the simplest type of fountain solution. Fountain solution isusually neutral to mildly acidic. However, for certain plates, mildlybasic fountain solution is used. The type of fountain solution useddepends on the type of the plate substrate as well as the photosensitivelayer. Various fountain solution compositions are described in U.S. Pat.Nos. 4,030,417 and 4,764,213.

Emulsion of ink and fountain solution is an emulsion formed from ink andfountain solution during wet lithographic printing process. Becausefountain solution (containing primarily water) and ink are not miscible,they do not form stable emulsion. However, emulsion of ink and fountainsolution can form during shearing, compressing, and decompressingactions by the rollers and cylinders, especially the ink rollers andplate cylinder, on a wet lithographic press. For wet press withintegrated inking system, ink and fountain solution are emulsified onthe ink rollers before transferred to the plate.

After removing the overcoat and before mounting on press for developmentwith ink and/or fountain solution, the plate is preferably inspected bythe operator under white room light. Such inspection is to visuallycheck any printout (visible images) such as colored printout or patternsof different refractive index of the photosensitive layer in the laserimaged areas, in order to ensure that the plate has been imaged and toidentify the specific plates (with specific image patterns). The imagedand overcoat-removed plate preferably has printout which is coloredprintout or patterns of different refractive index of the photosensitivelayer in the laser imaged areas, more preferably colored printout.

The imagewise laser exposure and the overcoat removal of this inventioncan be performed with the plate under any lightings (including darkness)at least for certain amount of time, as long as the exposure to suchlightings for such amount of time will not cause hardening of thephotosensitive layer; such lighting can be a yellow or red light (forpreferably limited time, more preferably less than 120 minutes, and mostpreferably less than 30 minutes), darkness or substantial darkness, orwhite light (for limited time, preferably less than 60 minutes, and morepreferably less than 20 minutes, and most preferably less than 5minutes). Preferably, the laser exposure is performed with the plateunder lightings (including darkness) that will not cause hardening ofthe photosensitive layer (even after long exposure); more preferably,under lightings containing no or substantially no radiation below awavelength selected from 400 to 650 nm (such as 500 nm), or in darknessor substantial darkness; and most preferably under lightings containingno radiation below a wavelength selected from 400 to 650 nm, or indarkness. The lighting containing no or substantially no radiation belowa wavelength selected from 400 to 650 nm (such as 500 nm) is usually ayellow or red light. This includes a light that is from a fluorescent orincandescent lamp covered with a filter that cuts off all orsubstantially all (at least 99%) of the radiation below a wavelengthselected from 400 to 650 nm; preferably the lamp is covered with afilter that cuts off all of the radiation below a wavelength selectedfrom 400 to 650 nm. The laser exposure and the overcoat removal can beperformed with the plate under the same or different lightings.

The plate can be imagewise exposed on a laser imager and then strippedoff the overcoat on an overcoat removal device. The imager and theovercoat removal device may stay open to the room light which is a whitelight (for limited time) or a yellow or red light (preferably forlimited time), preferably a yellow or red light, depending on thesensitivity of the plate. Preferably, the laser imager and/or theovercoat removal device are covered with light-tight covers which arenon-transparent or substantially non-transparent to the room light orare only transparent to yellow or red light, so that the plates are indarkness or substantial darkness or under yellow or red light while onthe imager and/or the overcoat removal device. More preferably, both theimager and the overcoat removal device are covered with light-tightcovers which are non-transparent or substantially non-transparent to theroom light or are only transparent to yellow or red light. Even morepreferably, both the imager and the overcoat removal device are coveredwith light-tight covers which consist of primarily non-transparent areasand small yellow or red light-passing-only areas; the small yellow orred light-passing-only areas allow monitoring the plate on the imager orovercoat removal device by the operator. Most preferably, both theimager and the overcoat removal device are covered with light-tightcovers which are non-transparent to the room light, so that no orsubstantially no room light reaches the plate during imaging andovercoat removal.

The plate can be manually or automatically handled between the imagerand the overcoat removal device. Preferably, the imager and the overcoatremoval device are connected (either directly or through a transportingmeans for transporting the plate from the imager to the overcoat removaldevice) and the imaged plate is automatically transported from theimager to the overcoat removal device. Most preferably, the imaged plateis automatically transported from the imager to the overcoat removaldevice, and the imager, overcoat removal device, and the connectionbetween them if not directly connected, are shielded with covers whichprevent all or substantially all of the room light or of the below-450nm portion of the room light from reaching the plate during imaging,transferring from the imager to the overcoat removal device, andovercoat removal.

The plate can be fed to the imager manually or automatically.Preferably, the plate is packaged in a light-tight or yellow or redlight-passing-only cassette (including any form of covered structurewhich is capable of storing two or more plates and feeding one plate ata time) to automatically feed to the imager. More preferably, the plateis automatically fed from a light-tight cassette to the imager, and theimaged plate is automatically transferred to the overcoat removaldevice. Most preferably, the plate is automatically fed from alight-tight cassette to the imager, the imaged plate is automaticallytransferred to the overcoat removal device, and both the imager and theovercoat removal device are shielded with light-tight covers, so thatthe plate is shielded from the room light during the feeding to theimager, laser imaging, transferring from the imager to the overcoatremoval device, and overcoat removal until the overcoat is removed; theroom lighting is preferably a white light.

This invention is further illustrated by the following examples of itspractice.

EXAMPLES 1-8

An electrochemically grained, anodized and polyvinyl phosphonic acidtreated aluminum substrate was coated with a thermosensitive layerformulation PS-1 with a #6 Meyer rod, followed by drying in an oven at100° C. for 2 min.

PS-1 Component Weight (g) Neocryl B-728 (Polymer from Zeneca) 2.50Ebecryl 220 (Acrylate monomer from UCB Chemicals) 5.99 Pluronic L43(Nonionic surfactant from BASF) 0.40(4-(2-Hydroxytetradecyl-oxy)phenyl)phenyliodonium 1.00hexafluorophosphate PINA KF-1151 (Infrared absorbing polymethine dyefrom 0.10 Allied Signal) 2-Butanone 90.00

The thermosensitive layer coated plate was further coated with awater-soluble overcoat OC-1 using a #6 Meyer rod, followed by hot airblow drying and baking in an oven at 100° C. for 2 min.

OC-1 Component Weight (g) Airvol 203 (polyvinyl alcohol from AirProducts) 4.84 Silwet 7604 (Surfactant from Union Carbide) 0.02 TritonX-100 (Surfactant from www.chemistrystore.com) 0.14 Water 95.00

The coated plate was exposed with an infrared laser imager equipped withlaser diodes emitting at about 830 nm (Trendsetter from Creo) at adosage of about 200 mJ/cm². The exposed areas of the plate showed browncolor, with the non-exposed areas remain light green. The exposed platewas cut into 8 pieces for further tests. Unless indicated otherwise, theplates were tested under a yellow light, and stored in a light-tight boxbetween tests.

The 1^(st) piece was stripped off the overcoat with a pressure sensitiveadhesive tape (Scotch Brand packaging tape from 3M). The adhesive tapewas taped to the coated side of the plate, and then pulled away from theplate. The overcoat in the taped areas came out with the tape and wasremoved from the plate. The same procedure was repeated for the restareas of the plate, until the overcoat in the whole plate piece wasremoved. The 2^(nd), 3^(rd) and 4^(th) pieces were stripped off theovercoat with the same procedure as for the 1^(st) piece. The 5^(th),6^(th), 7^(th) and 8^(th) pieces were not stripped.

The plate pieces were then exposed to an office white fluorescent light(total of 80 watts at about 2 meters) for various amounts of time aslisted in Table 1, to test white light stability.

Each of the above plate pieces was tested on a wet lithographic press ABDick 360. The press tests were performed under a white fluorescentlight. Each plate was exposed to the white fluorescent light for about 5minutes during the handling, mounting, and on-press development beforebeing developed with ink and/or fountain solution The plate was directlymounted on the plate cylinder of the press. After starting the press,the fountain roller was engaged for 20 rotations, the ink roller(carrying emulsion of ink and fountain solution) was applied to theplate cylinder for 20 rotations, and the plate cylinder was then engagedwith the blanket cylinder and printed with paper for 200 impressions.The printed sheets were evaluated for on-press developability of theplate, with the results summarized in Table 1.

TABLE 1 Performance on AB Dick 360 lithographic press ExposureBackground Inking in Overcoat time to white Background at at 200 imagingStripping room light 20 impressions impressions areas Stripped NoneClean Clean Good Stripped  30 minutes Clean Clean Good Stripped 120minutes Clean Clean Good Stripped 300 minutes Clean Clean GoodNon-stripped None Clean Clean Good Non-stripped  30 minutes Slightlytoning Clean Good Non-stripped 120 minutes Toning Toning GoodNon-stripped 300 minutes Heavy toning Heavy toning Good

EXAMPLES 9-14

The plate as prepared in EXAMPLES 1-8 was exposed with an infrared laserimager equipped with laser diodes emitting at about 830 nm (Trendsetterfrom Creo) at a dosage of about 200 mJ/cm². The exposed areas of theplate showed brown color, with the non-exposed areas remain light green.The exposed plate was cut into 6 pieces for further tests.

The first piece was dipped in a 10% by weight citric acid aqueoussolution for 20 seconds. The second piece was dipped in a 10% by weightsodium chloride aqueous solution for 20 seconds. The third piece wasdipped in a 0.2% by weight sodium hydroxide aqueous solution for 20seconds. The fourth piece was dipped in a 10% by weight sodiumxylenesulfonate aqueous solution for 20 seconds. The fifth piece wasrinsed with water. All the above pieces of plate were dried by hot airblow to remove any excess water. The treatments were performed underyellow light. The sixth piece was not treated. All the above platepieces were then exposed to an office white fluorescent light (total of80 watts at about 2 meters) for 2 hours. The appearances of the platepieces were listed in Table 2.

Each of the treated plates was tested on a wet lithographic press ABDick 360 (under office white fluorescent light). The plate was directlymounted on the plate cylinder of the press. After starting the press,the fountain roller was engaged for 20 rotations, the ink roller(carrying emulsion of ink and fountain solution) was applied to theplate cylinder for 20 rotations, and the plate cylinder was then engagedwith the blanket cylinder and printed with paper for 200 impressions.The printed sheets were evaluated for on-press developability of theplate, with the results summarized in Table 2.

TABLE 2 Appearance after laser Performance on AB Dick 360 exposure andtreatment lithographic press Non- Laser Background Background Inking inAqueous solution for exposed exposed at 20 at 200 imaging treating plateareas areas impressions impressions areas 10% Citric acid Light greenBrown Clean Clean Good 10% Sodium chloride Light green Brown Clean CleanGood 0.2% Sodium hydroxide Light green Light brown Clean Clean Good 10%Sodium Light green Light brown Clean Clean Good xylenesulfonate Rinsewith water Light green Brown Toning Toning Good No treatment Light greenBrown Inked Inked Good

EXAMPLE 15-18

An electrochemically roughened, anodized, and phosphate fluoride treatedaluminum sheet was first coated with a 0.1% aqueous solution ofpolyvinyl alcohol (Airvol 540 from Air Products) with a #6 Meyer rod,followed by drying in an oven at 100° C. for 2 min. The polyvinylalcohol coated substrate was further coated with the thermosensitivelayer formulation PS-2 with a #6 Meyer rod, followed by hot air blowdrying and baking in an oven at 100° C. for 2 min.

PS-2 Weight Component ratios Neocryl B-728 (Polymer from Zeneca) 2.73Sartomer SR-399 (Acrylic monomer from Sartomer) 6.52 Pluronic L43(Nonionic surfactant from BASF) 0.562,4-Bis(trichloromethyl)-6-[(4-ethoxyethylenoxy)-phen-1-yl]-s- 1.00triazine ADS-830AT (Infrared absorbing cyanine dye from American 0.10Dye Source) Acetone 90.0

The thermosensitive layer coated plate was further coated with awater-soluble overcoat OC-2 using a #18 Meyer rod, followed by hot airblow drying and baking in an oven at 100° C. for 2 min.

OC-2 Weight Component ratios Airvol 205 (Polyvinyl alcohol from AirProducts) 10.0 Triton X-100 (Surfactant from Union Carbide) 0.20 Water90.0

The plate was exposed with an infrared laser imager equipped with laserdiodes emitting at about 830 nm (Trendsetter from Creo). The plate wasmounted on the imaging drum and exposed at a laser dosage of about 300mJ/cm². The exposed areas showed glossier but essentially colorlessimaging pattern, in contrast to the less glossy non-imaged areas. Theexposed plate was cut into 4 pieces for further tests. Unless indicatedotherwise, the plates were tested under a yellow light, and stored in alight-tight box between tests.

The 1^(st) piece was stripped off the overcoat. A pressure sensitiveadhesive tape (Scotch Brand packaging tape from 3M) was taped onto theovercoat of the plate on one end, and then gently pulled away.Substantially all the overcoat of the plate piece came off as one singlefilm. The 2^(nd) and 3^(rd) pieces were stripped off the overcoat withthe same procedure as the 1^(st) piece. The 4^(th) piece was notstripped.

The 2^(nd) and 3^(rd) plate pieces were then exposed to an office whitefluorescent light (total of 80 watts at about 2 meters) for 30 and 120minutes respectively. The 1^(st) and 4^(th) plate pieces were notexposed to the office white light. The plate was then tested on pressfor development with ink and fountain solution and for lithographicprinting.

Each of the above plates was tested on a wet lithographic press AB Dick360. The press tests were performed under a white fluorescent light.Each plate was exposed to the white fluorescent light for about 5minutes during the handling, mounting, and on-press development beforebeing developed with ink and/or fountain solution. The plate wasdirectly mounted on the plate cylinder of the press. After starting thepress, the fountain roller was engaged for 20 rotations, the ink roller(carrying emulsion of ink and fountain solution) was applied to theplate cylinder for 20 rotations, and the plate cylinder was then engagedwith the blanket cylinder and printed with paper for 200 impressions.The 1^(st), 2^(nd) and 3^(rd) plate pieces (with overcoat stripped off)showed clean background under 20 impressions, good inking, and nowearing at 200 impressions. The 4^(th) plate piece (with overcoat notstripped) could not be developed on press, with the overcoat remainingon the plate after contacting with ink and fountain solution for 200impressions.

1. A method of lithographically printing images on a receiving medium,comprising in order: (a) providing a lithographic plate comprising (i) asubstrate, (ii) a photosensitive layer, and (iii) an overcoat; saidphotosensitive layer being soluble or dispersible in ink and/or fountainsolution and capable of hardening upon exposure to a laser having awavelength selected from 200 to 1200 nm; (b) imagewise exposing saidplate with said laser on a laser imager to cause hardening of saidphotosensitive layer in the exposed areas; (c) removing said overcoatfrom said plate; (d) mounting said plate on a lithographic press; and(e) contacting said plate with ink and/or fountain solution on saidpress to remove the photosensitive layer in the non-hardened areas andto lithographically print images from said plate to the receivingmedium; (f) wherein said steps (d) to (e) are performed with the plateunder a white room light, said photosensitive layer before the removalof the overcoat is capable of hardening or causing background toningunder said room light for less than a time period selected from 1 to 60minutes, and said photosensitive layer after the removal of the overcoatis incapable of hardening or causing background toning under said roomlight for at least twice of said time period.
 2. The method of claim 1wherein said photosensitive layer before the removal of the overcoat iscapable of hardening or causing background toning under said room lightfor less than 60 minutes, and said photosensitive layer after theremoval of the overcoat is incapable of hardening or causing backgroundtoning under said room light for at least 120 minutes.
 3. The method ofclaim 1 wherein said photosensitive layer before the removal of theovercoat is capable of hardening or causing background toning under saidroom light for less than 30 minutes, and said photosensitive layer afterthe removal of the overcoat is incapable of hardening or causingbackground toning under said room light for at least 60 minutes.
 4. Themethod of claim 1 wherein said photosensitive layer before the removalof the overcoat is capable of hardening or causing background toningunder said room light for less than 10 minutes, and said photosensitivelayer after the removal of the overcoat is incapable of hardening orcausing background toning under said room light for at least 60 minutes.5. The method of claim 1 wherein said photosensitive layer before theremoval of the overcoat is capable of hardening or causing backgroundtoning under said room light for less than 1 minute, and saidphotosensitive layer after the removal of the overcoat is incapable ofhardening or causing background toning under said room light for atleast 60 minutes.
 6. The method of claim 1 wherein the total exposuretime of said plate to said room light in said steps (d) to (e) beforethe plate is developed is less than twice of said time period.
 7. Themethod of claim 1 wherein said overcoat is removed by mechanicalpeeling.
 8. The method of claim 1 wherein said overcoat is soluble ordispersible in water or an aqueous solution and is removed by rinsingwith water or an aqueous solution.
 9. The method of claim 8 wherein saidrinsed plate is dried with forced air to dry off any wet-looking liquidlayer on the plate surface before mounting on press.
 10. The method ofclaim 1 wherein said overcoat is soluble or dispersible in water and isremoved by rinsing with water.
 11. The method of claim 1 wherein saidovercoat is soluble or dispersible in water and is removed by rinsingwith an aqueous solution, and said aqueous solution contains adeactivating agent and is capable of deactivating said photosensitivelayer in the non-hardened areas.
 12. The method of claim 1 wherein saidovercoat is soluble or dispersible in water and is removed by rinsingwith an aqueous solution, and said aqueous solution contains ahydrophilizing agent and is capable of improving the hydrophilicity ofthe substrate.
 13. The method of claim 1 wherein said plate is visuallyinspected by an operator under white room light after removing theovercoat and before mounting on press.
 14. The method of claim 1 whereinsaid plate is stacked together with one or more other plates afterovercoat removal and before mounting on press for development with inkand/or fountain solution; provided that, for plate which is rinsed withwater or an aqueous solution to remove the overcoat, the rinsed plate isfurther dried to remove any wet-looking surface layer before stacking.15. The method of claim 1 wherein said overcoat has a coverage of atleast 1.0 g/m².
 16. The method of claim 1 wherein said overcoat isincapable of being completely removed with ink and/or fountain solutionon said lithographic press after contacting with the ink roller and/orfountain solution roller for up to 200 rotations of the plate cylinder.17. The method of claim 1 wherein said photosensitive layer comprises apolymeric binder, a free radical polymerizable monomer, a free radicalinitiator, and a sensitizing dye.
 18. The method of claim 1 wherein saidlaser is a violet or ultraviolet laser having a wavelength selected from200 to 430 nm.
 19. The method of claim 1 wherein said laser is aninfrared laser having a wavelength selected from 750 to 1200 nm.
 20. Themethod of claim 1 wherein said steps (b) to (c) are performed with theplate under a yellow or red light, or in darkness or substantialdarkness.
 21. The method of claim 1 wherein said overcoat removal (stepc) is performed on an overcoat removal device that is connected to or ispart of an imaging device for said laser imaging (step b).
 22. Themethod of claim 21 wherein both said imaging device and said overcoatremoval device are shielded with covers which prevent at least 99% ofroom light from reaching said plate on said device.
 23. A method oflithographically printing images on a receiving medium, comprising inorder: (a) providing a lithographic plate comprising (i) a hydrophilicsubstrate, (ii) an oleophilic photosensitive layer comprising apolymeric binder, a free radical polymerizable monomer, a free radicalinitiator, and a sensitizing dye, and (iii) an overcoat; saidphotosensitive layer being soluble or dispersible in ink and/or fountainsolution and capable of hardening upon exposure to a laser having awavelength selected from 200 to 1200 nm; (b) imagewise exposing saidplate with said laser on a laser imager to cause hardening of saidphotosensitive layer in the exposed areas; (c) removing said overcoatfrom said plate; (d) visually inspecting said plate by an operator; (e)mounting said plate on a lithographic press; and (f) contacting saidplate with ink and/or fountain solution on said press to remove thephotosensitive layer in the non-hardened areas and to lithographicallyprint images from said plate to the receiving medium; (g) wherein saidsteps (d) to (f) are performed with the plate under a white room light,said photosensitive layer before the removal of the overcoat is capableof hardening or causing background toning under said room light for lessthan 30 minutes, and said photosensitive layer after the removal of theovercoat is incapable of hardening or causing background toning undersaid room light for at least 60 minutes.
 24. A method oflithographically printing images on a receiving medium, comprising inorder: (a) providing a lithographic plate comprising (i) a hydrophilicsubstrate, (ii) an oleophilic photosensitive layer comprising apolymeric binder, a free radical polymerizable monomer, a free radicalinitiator, and a sensitizing dye, and (iii) an water soluble ordispersible overcoat; said photosensitive layer being soluble ordispersible in ink and/or fountain solution and capable of hardeningupon exposure to a laser having a wavelength selected from 200 to 1200nm; (b) imagewise exposing said plate with said laser on a laser imagerto cause hardening of said photosensitive layer in the exposed areas;(c) removing said overcoat from said plate by rinsing with water or anaqueous solution; (d) drying said rinsed plate with forced hot air todry off any wet-looking liquid layer on the plate surface; (e) mountingsaid plate on a lithographic press; and (f) contacting said plate withink and/or fountain solution on said press to remove the photosensitivelayer in the non-hardened areas and to lithographically print imagesfrom said plate to the receiving medium; (g) wherein said steps (e) to(f) are performed with the plate under a white room light, saidphotosensitive layer before the removal of the overcoat is capable ofhardening or causing background toning under said room light for lessthan 30 minutes, and said photosensitive layer after the removal of theovercoat is incapable of hardening or causing background toning undersaid room light for at least 60 minutes.